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ANALOG DEVICES AD7888 +2.7 V to +5.25 V Micropower 8-Channel 125 kSPS 12-Bit ADC in 16-Lead TSSOP handbook

1. 93 2 65 12 85 23 15 33 65 43 85 54 35 64 15 INPUT FREQUENCY kHz Figure 4 PSRR vs Frequency CIRCUIT INFORMATION The AD7888 is a fast low power 12 bit single supply 8 channel A D converter The part can be operated from 3 V 2 7 V to 3 6 V supply or from 5 V 4 75 V to 5 25 V supply When operated from either a 5 V supply or a 3 V supply the AD7888 is capable of throughput rates of 125 kSPS when provided with a 2 MHz clock The AD7888 provides the user with an 8 channel multiplexer on chip track hold A D converter reference and serial interface housed in a tiny 16 lead TSSOP package which offers the user considerable space saving advantages over alternative solutions The serial clock input accesses data from the part and also provides the clock source for the successive approximation A D converter The analog input range is 0 to Vggr where the externally applied Vpzr can be between 1 2 V and Vpp The 8 channel multiplexer is controlled by the part s Control Register This Control Register also allows the user to power off the internal reference and to determine the Modes of Operation CONVERTER OPERATION The AD7888 is a successive approximation analog to digital converter based around a charge redistribution DAC Figures 5 and 6 show simplified schematics of the ADC Figure 5 shows the ADC during its acquisition phase SW2 is closed and SW1 is in Position A the comparator is held in a balanced condi
2. 1 2 V to Vpp 3 Vpp Power Supply Input The Vpp range for the AD7888 is from 2 7 V to 5 25 V 4 13 AGND Analog Ground Ground reference point for all circuitry on the AD7888 All analog input signals and any external reference signals should be referred to this AGND voltage Both of these pins should connect to the AGND plane of a system 5 12 AINI AIN8 Analog Input 1 through Analog Input 8 Eight single ended analog input channels that are multiplexed into the on chip track hold The analog input channel to be converted is selected by using the ADDO through ADD2 bits of the Control Register The input range for all input channels is 0 to Vagr Any unused input channels should be connected to AGND to avoid noise pickup 14 DIN Data In Logic Input Data to be written to the AD7888 s Control Register is provided on this input and is clocked into the register on the rising edge of SCLK see Control Register section 15 DOUT Data Out Logic Output The conversion result from the AD7888 is provided on this output as a serial data stream The bits are clocked out on the falling edge of the SCLK input The data stream consists of four leading zeros followed by the 12 bits of conversion data which is provided MSB first 16 SCLK Serial Clock Logic Input SCLK provides the serial clock for accessing data from the part and writing serial data to the Control Register This clock input is also used as the clock source for the AD7888 s conversion proces
3. 1 AND PMO 0 THE PART REMAINS POWERED UP AS PM1 AND PMO 0 4 LEADING ZEROES CONVERSION RESULT DATA IN PM1 1 AND PMO 0 TO PLACE THE PART BACK IN AUTOSHUTDOWN MODE Figure 14b Autoshutdown Operation CS and the second rising edge of SCLK as shown in Figure 14a In microcontroller applications this is readily achievable by driving the CS input from one of the port lines and ensuring that the serial data read from the microcontrollers serial port is not initiated for 5 us In DSP applications where the CS is generally derived from the serial frame synchronization line it is not possible to separate the first falling edge and second rising edge of SCLK after the CS falling edge by up to 5 us There fore the user will need to write to the Control Register to exit this mode and by writing PM1 0 and PMO 0 put the part into normal mode A second conversion will then need to be initiated when the part is powered up to obtain a conversion result as shown in Figure 14b THE PART ENTERS STANDBY AT THE END OF CONVERSION AS PM1 1 AND PMO 1 Autostandby PM1 1 PMO 1 In this mode the AD7888 automatically enters a standby or sleep mode at the end of every conversion In this standby mode all on chip circuitry apart from the on chip reference is powered down This mode is similar to the autoshutdown but in this case the power up time is much shorter as the on chip reference remains powered up at all
4. Input Impedance 5 5 kQ typ Very High Impedance If Internal Reference Disabled REFOUT Output Voltage 2 45 2 55 2 45 2 55 V min max REFOUT Tempco 50 50 ppm C typ LOGIC INPUTS Input High Voltage Ving 2 4 2 4 V min Vpp 4 75 V to 5 25 V 2 1 2 1 V min Vpp 2 7 V to 3 6 V Input Low Voltage Vin 0 8 0 8 V max Vpp 2 7 V to 5 25 V Input Current Iy 10 10 uA max Typically 10 nA Vm 0 V or Vpp Input Capacitance Cpt 10 10 pF max LOGIC OUTPUTS Output High Voltage Voy Isource 200 HA Vpp 0 5 Vpp 0 5 V min Vpp 2 7 V to 5 25 V Output Low Voltage Vor 0 4 0 4 V max Ismx 200 uA Floating State Leakage Current 10 10 uA max Floating State Output Capacitance 10 10 pF max Output Coding Straight Natural Binary CONVERSION RATE Throughput Time 16 16 SCLK Cycles Conversion Time Acquisition Time 125 kSPS with 2 MHz Clock Track Hold Acquisition Time 1 5 1 5 SCLK Cycles Conversion Time 14 5 14 5 SCLK Cycles 7 25 us 2 MHz Clock 2 REV 0 AD7888 Parameter A Version B Version Units Test Conditions Comments POWER REQUIREMENTS Vpp 2 7 5 25 2 7 5 25 V min max Ipp 7 Normal Mode Static 700 700 uA max Normal Mode Operational 700 700 HA typ fsampLe 125 kSPS Using Standby Mode 450 450 uA typ fsampLe 50 kSPS Using Shutdown Mode 80 80 uA typ fsamp_Le 10 kSPS 12 12 uA typ fsampLe 1 KSPS Standby Mode 200 200 uA max Vpp 2 7 V to 5 25 V Shutdown Mode 2 2 uA max Vpp 4
5. designators ESD electrostatic discharge sensitive device Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection Although the AD7888 features proprietary ESD protection circuitry permanent damage may occur on devices subjected to high energy electrostatic discharges Therefore proper ESD precautions are recommended to avoid performance degradation or loss of functionality WARNING ee ESD SENSITIVE DEVICE REV 0 3 AD7888 TIMING SPECIFICATIONS Ta Twin to Tmax unless otherwise noted Limit at Tins Tmax A B Versions Parameter 4 75 V to 5 25 V 2 7 V to 3 6 V Units Description Reape 2 2 MHz max tCONVERT 14 5 tscik 14 5 tscik tacaq 1 5 tscLK 1 5 tsc K Throughput Time tconvert taco 16 tsctx ty 10 10 ns min CS to SCLK Setup Time t 30 60 ns max Delay from CS until DOUT 3 State Disabled t 75 100 ns max Data Access Time after SCLK Falling Edge ty 20 20 ns min Data Setup Time Prior to SCLK Rising Edge t5 20 20 ns min Data Valid to SCLK Hold Time te 0 4 tscLK 0 4 tsc K ns min SCLK High Pulsewidth t7 0 4 tscLK 0 4 tsc K ns min SCLK Low Pulsewidth ts 80 80 ns max CS Rising Edge to DOUT High Impedance to 5 5 us typ Power Up Time from Shutdown NOTES Sample tested at 25 C to ensure compliance All input signals are specified with tr tf 5 ns 10 to 90 of V pp and timed from a voltage
6. nominal value _6 REV 0 AD7888 CONTROL REGISTER The Control Register on the AD7888 is an 8 bit write only register Data is loaded from the DIN pin of the AD7888 on the rising edge of SCLK The data is transferred on the DIN line at the same time as the conversion result is read from the part This requires 16 serial clocks for every data transfer Only the information provided on the first 8 rising clock edges after CS falling edge is loaded to the Control Register MSB denotes the first bit in the data stream The bit functions are outlined in Table I The default contents of the Control Register on power up is all zeros Table I Control Register Bit Function Description MSB DONTC ZERO ADD2 ADD1 ADDO REF PM1 PMO Bit Mnemonic Comment 7 DONTC Don t Care The value written to this bit of the Control Register is a don t care i e it doesn t matter if the bit is Oorl 6 ZERO A zero must be written to this bit to ensure correct operation of the AD7888 5 ADD2 These three address bits are loaded at the end of the present conversion sequence and select which analog input 4 ADD1 channel is converted for the next conversion The selected input channel is decoded as shown in Table II 3 ADDO 2 REF Reference Bit With a 0 in this bit the on chip reference is enabled With a 1 in this bit the on chip reference is disabled To obtain best performance from the AD7888 the internal reference should be disabled when
7. the write during that conversion To continue to operate in this mode the user must ensure that PM1 and PMO are both loaded with 0 on every data transfer The falling edge of CS initiates the sequence and the input signal is sampled on the second rising edge of the SCLK input Sixteen serial clock cycles are required to complete the conver sion and access the conversion result Once a data transfer is complete CS has returned high another conversion can be initiated immediately by bringing CS low again THE PART ENTERS SHUTDOWN AT THE END OF CONVERSION AS PM1 1 AND PMO 0 SCLK u u DOUT 4 LEADING ZEROES CONVERSION RESULT DATA IN CONTROL REGISTER DATA IS LOADED ON THE FIRST 8 CLOCKS PM1 1 AND PMO 0 DIN Full Shutdown PM1 0 PMO 1 In this mode all internal circuitry on the AD7888 including the on chip reference is powered down The part retains the infor mation in the Control Register during full shutdown The part remains in full shutdown until the power management bits are changed If the power management bits are changed to PM1 1 and PMO 0 i e the autoshutdown mode the part will remain in shutdown now in autoshutdown but will power up once a conversion is initiated after that see Power Up Times section The part changes mode as soon as the control register has been updated so if the part is in full shutdown mode and the power management bits are changed to PM1 PMO 0 i e normal mo
8. the second rising edge of the SCLK input after the falling edge of CS Thus the first one and one half clock cycles after the falling edge of CS is when the acquisition of the input signal takes place This time is denoted as the acquisition time taco In autoshutdown mode the acquisition time must allow for the wake up time of 5 us The on chip track hold goes from track mode to hold mode on the second rising edge of SCLK and a conversion is also initiated on this edge The conversion process takes a further fourteen and one half SCLK cycles to complete The rising edge of CS will put the bus back into three state If CS is left low a new conver sion will be initiated The input channel that is sampled is the one selected in the previous write to the Control Register Thus the user must write ahead of the channel for conversion In other words the user must write the channel address for the next conversion while the present conversion is in progress Writing of information to the Control Register takes place on the first eight rising edges of SCLK in a data transfer The Con trol Register is always written to when a data transfer takes place The user must be careful to always set up the correct information on the DIN line when reading data from the part Sixteen serial clock cycles are required to perform the conver sion process and to access data from the AD7888 In applica tions where the first serial clock edge following CS
9. times Figure 15 shows the general diagram of the operation of the AD7888 in this mode On the first falling SCLK edge after CS goes low the AD7888 comes out of standby The AD7888 wake up time is very short in this mode so it is possible to wake up the part and carry out a valid conversion in the same read write operation The input signal is sampled on the second rising edge of SCLK following the CS falling edge At the end of conversion last rising edge of SCLK the part automatically enters its standby mode THE PART POWERS UP FROM STANDBY ON SCLK FALLING EDGE AS PM1 1 AND PMO 1 SCLK u u DOUT 4 LEADING ZEROES CONVERSION RESULT DATA IN CONTROL REGISTER DATA IS LOADED ON THE FIRST 8 CLOCKS PM1 1 AND PMO 1 DIN u u 4 LEADING ZEROES CONVERSION RESULT DATA IN PM1 1 AND PMO 1 TO KEEP THE PART IN THIS MODE Figure 15 Autostandby Operation 2 REV 0 AD7888 SERIAL INTERFACE Figure 16 shows the detailed timing diagram for serial interfac ing to the AD7888 The serial clock provides the conversion clock and also controls the transfer of information to and from the AD7888 during conversion CS initiates the data transfer and conversion process For the autoshutdown mode the first falling edge of SCLK after the falling edge of CS wakes up the part In all cases it gates the serial clock to the AD7888 and puts the on chip track hold into track mode The input signal is sampled on
10. 75 V to 5 25 V 0 5 uA typ 1 1 uA max Vpp 2 7 V to 3 6 V Normal Mode Power Dissipation 3 5 3 5 mW max Vpp 5 V 2 1 2 1 mW max Vpp 3 V Shutdown Power Dissipation 10 10 uW max Vpp 5 V 3 3 uW max Vpp 3 V Standby Power Dissipation 1 1 mW max Vpp 5 V 600 600 uW max Vpp 3 V NOTES Temperature ranges as follows A Version 40 C to 105 C B Version 0 C to 105 C See Terminology 3SNR calculation includes distortion and noise components 4Sample tested 25 C to ensure compliance SAll digital inputs GND except CS Vpp No load on the digital outputs Analog inputs GND SCLK GND when SCLK off All digital inputs GND except for CS Vpp No load on the digital outputs Analog inputs GND Specifications subject to change without notice ABSOLUTE MAXIMUM RATINGS Ta 25 C unless otherwise noted VDD tO AGND riiai a eniai detente 0 3 V to 7 V Analog Input Voltage to AGND 0 3 V to Vpp 0 3 V Digital Input Voltage to AGND 0 3 V to Vpp 0 3 V Digital Output Voltage to AGND 0 3 V to Vpp 0 3 V REFIN REFOUT to AGND 0 3 V to Vpp 0 3 V Input Current to Any Pin Except Supplies 10 mA Operating Temperature Range Commercial A Version secede a a Eee 40 C to 105 C B Version is hase a EEA 0 C to 105 C Storage Temperature Range 65 C to 150 C Junction Temperature 0 00 e eee 150 C SOIC TSSOP Package Power Diss
11. 88 should be sunk in the AGND plane The AGND plane and DGND plane connection should be made at one point only a star ground point that should be established as close as possible to an AGND pin of the AD7888 Avoid running digital lines under the device as these will couple noise onto the die The analog ground plane should be allowed to run under the AD7888 to avoid noise coupling The power supply lines to the AD7888 should use as large a trace as pos sible to provide low impedance paths and reduce the effects of glitches on the power supply line Fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other sections of the board and clock signals should never be run near the analog inputs Avoid crossover of digital and analog signals Traces on opposite sides of the board should run at right angles to each other This will reduce the effects of feedthrough through the board A microstrip technique is by far the best but is not always possible with a double sided board In this technique the component side of the board is dedicated to ground planes while signals are placed on the solder side Good decoupling is also important All analog supplies should be decoupled with 10 uF tantalum in parallel with 0 1 uF ca pacitors to AGND To achieve the best from these decoupling components they must be placed as close as possible to the device ideally right up against the device Evaluating the AD7
12. 888 Performance The recommended layout for the AD7888 is outlined in the evaluation board for the AD7888 The evaluation board pack age includes a fully assembled and tested evaluation board documentation and software for controlling the board from the PC via the EVAL CONTROL BOARD The EVAL CONTROL BOARD can be used in conjunction with the AD7888 evalua tion board as well as many other Analog Devices evaluation boards ending in the CB designator to demonstrate evaluate the ac and dc performance of the AD7888 The software allows the user to perform ac fast Fourier trans form and dc histogram of codes tests on the AD7888 15 AD7888 OUTLINE DIMENSIONS Dimensions shown in inches and mm 16 Lead Small Outline SOIC R 16A 0 3937 10 00 0 3859 9 80 J f 0 2440 6 20 0 2284 5 80 bk aq PIN 1 0 0688 1 75 0 0196 0 50 45o 0 0098 0 25 0 0532 1 35 0 0099 0 25 5 0 0040 0 10 is Jy ve gt e gt ii o f 0 0500 0 0192 0 49 PEOS olhe 0 0099 0 25 0 0500 1 27 SEATING a 27 0 0075 0 19 0 0160 0 41 PLANE 0 0138 0 35 16 Lead Thin Shrink Small Outline TSSOP RU 16 Tiina 2 0 177 4 50 0 169 4 30 0 256 6 50 0 246 6 25 2 0 006 0 15 0 002 0 05 0 0433 RET FA eee aaa F g 0 028 0 70 fja 11 SEATING 085 0 0119 0 90 2 9118 050 0 020 0 50 PLANE 0 0075 0 19 0 007
13. 9 0 20 0 0035 0 090 16 REV 0 C3450 8 2 99 PRINTED IN U S A
14. AGRAM Figure 8 shows a typical connection diagram for the AD7888 Both AGND pins are connected to the analog ground plane of the system Vpgr is connected to a well decoupled Vpp pin to provide an analog input range of 0 V to Vpp The conversion result is output in a 16 bit word with four leading zeroes fol lowed by the MSB of the 12 bit result For applications where power consumption is of concern the automatic power down at the end of conversion should be used to improve power perfor mance See Modes of Operation section of the data sheet SUPPLY 2 7V O TO 5 25V SERIAL INTERFACE REF IN REF OUT AD7888 Figure 8 Typical Connection Diagram Analog Input Figure 9 shows an equivalent circuit of the analog input structure of the AD7888 The two diodes D1 and D2 provide ESD pro tection for the analog inputs Care must be taken to ensure that the analog input signal never exceeds the supply rails by more than 200 mV This will cause these diodes to become forward biased and start conducting current into the substrate 20 mA is the maximum current these diodes can conduct without causing irreversible damage to the part However it is worth noting that a small amount of current 1 mA being conducted into the substrate due to an overvoltage on an unselected channel can cause inaccurate conversions on a selected channel The capaci tor C1 in Figure 9 is typically about 4 pF and can primarily be attributed to pin capacitan
15. O 0 AD7888 0 O ANALOG 2 7V to 5 25 V Micropower 8 Channel DEVICES 125 kSPS 12 Bit ADC in 16 Lead TSSOP AD7888 FEATURES FUNCTIONAL BLOCK DIAGRAM Specified for Vpp of 2 7 V to 5 25 V Flexible Power Throughput Rate Management Shutdown Mode 1 pA Max AIN1 AD7888 Eight Single Ended Inputs d Serial Interface SPI QSPI MICROWIRE DSP AIN8 DD Compatible 16 Lead Narrow SOIC and TSSOP Packages APPLICATIONS Battery Powered Systems Personal Digital Assistants Medical Instruments Mobile Communications Instrumentation and Control Systems High Speed Modems REF IN REF OUT Q GENERAL DESCRIPTION AGND O AGND The AD7888 is a high speed low power 12 bit ADC that oper ates from a single 2 7 V to 5 25 V power supply The AD7888 is capable of a 125 kSPS throughput rate The input track and O O O hold acquires a signal in 500 ns and features a single ended sampling scheme The AD7888 contains eight single ended analog inputs AIN1 through AIN8 The analog input on each PRODUCT HIGHLIGHTS of these channels is from 0 to Vggr The part is capable of con 1 Smallest 12 bit 8 channel ADC 16 lead TSSOP is the same verting full power signals up to 2 5 MHz area as an 8 lead SOIC and less than half the height The AD7888 features an on chip 2 5 V reference that can be 2 Lowest Power 12 bit 8 channel ADC used as the reference source for the A D converter The REF IN REF OUT pin allows the user acce
16. UGHPUT RATE By operating the AD7888 in autoshutdown or autostandby mode the average power consumption of the AD7888 decreases at lower throughput rates Figure 12 shows how as the through put rate is reduced the device remains in its power down state longer and the average power consumption over time drops accordingly For example if the AD7888 were operated in a continuous sampling mode with a throughput rate of 10 kSPS and a SCLK of 2 MHz Vpp 5 V and if PM1 1 and PMO 0 i e the device is in autoshutdown mode and the on chip reference is used the power consumption is calculated as follows The power dissipation during normal operation is 3 5 mW Vpp 5 V If the power up time is 5 us and the remaining conversion plus acquisition time is 15 5 tscyx i e approximately 7 75 us see Figure 14a the AD7888 can be said to dissipate 3 5 mW for 12 75 us during each conversion cycle If the throughput rate is 10 kSPS the cycle time is 100 us and the average power dissi pated during each cycle is 12 75 100 x 3 5 mW 446 25 uW If Vpp 3 V SCLK 2 MHz and the device is again in auto shutdown mode using the on chip reference the power dissipa tion during normal operation is 2 1 mW The AD7888 can now be said to dissipate 2 1 mW for 12 75 us during each conversion cycle With a throughput rate of 10 kSPS the average power dissipated during each cycle is 12 75 100 x 2 1 mW 267 75 uW Figure 12 shows the power vs thro
17. achieved 14 The Timer Registers etc are loaded with a value that will provide an interrupt at the required sample interval When an interrupt is received a value is transmitted with TFS DT ADC control word The TFS is used to control the RFS and hence the reading of data The frequency of the serial clock is set in the SCLKDIV Register When the instruction to transmit with TES is given i e AXO TX0 the state of the SCLK is checked The DSP will wait until the SCLK has gone high low and high before transmission will start If the timer and SCLK values are chosen such that the instruction to transmit occurs on or near the rising edge of SCLK the data may be transmitted or it may wait until the next clock edge For example the ADSP 2111 has a master clock frequency of 16 MHz If the SCLKDIV Register is loaded with the value 3 a SCLK of 2 MHz is obtained and eight master clock periods will elapse for every one SCLK period If the timer registers are loaded with the value 803 then 100 5 SCLKs will occur between interrupts and subsequently between transmit instructions The situation will result in nonequidistant sampling as the transmit instruction is occurring on a SCLK edge If the number of SCLKs between interrupts is not a figure of N 5 equidistant sampling will be implemented by the DSP AD7888 ADSP 21xx ADDITIONAL PINS OMITTED FOR CLARITY Figure 18 Interfacing to the ADSP 21xx AD7888 to DSP56xxx T
18. ce The resistor R1 is a lumped com ponent made up of the on resistance of a multiplexer and a switch This resistor is typically about 100 Q The capacitor C2 is the ADC sampling capacitor and has a capacitance of 20 pF typically Note The analog input capacitance seen when the track and hold is in track mode is typically 38 pF while in hold mode it is typically 4 pF C2 R1 20pF Vin w RK CONVERSION PHASE SWITCH OPEN TRACK PHASE SWITCH CLOSED Figure 9 Equivalent Analog Input Circuit REV 0 For ac applications removing high frequency components from the analog input signal is recommended by use of an RC low pass filter on the relevant analog input pin In applications where harmonic distortion and signal to noise ratio are critical the analog input should be driven from a low impedance source Large source impedances will significantly affect the ac perfor mance of the ADC This may necessitate the use of an input buffer amplifier The choice of the op amp will be a function of the particular application When no amplifier is used to drive the analog input the source impedance should be limited to low values The maximum source impedance will depend on the amount of total harmonic distortion THD that can be tolerated The THD will increase as the source impedance increases and performance will degrade Figure 10 shows a graph of the total harmonic distortion versus analog input signal frequency for different s
19. de then the part will power up on the 16th SCLK rising edge Autoshutdown PM1 1 PMO 0 In this mode the AD7888 automatically enters its power down mode at the end of every conversion Figure 14a shows the general diagram of the operation of the AD7888 in this mode When CS goes from high to low all on chip circuitry will start to power up on the next falling edge of SCLK On the sixteenth SCLK rising edge the part will power down again It takes ap proximately 5 us for the AD7888 internal circuitry to be fully powered up As a result a conversion or sample and hold acquisition should not be initiated during this 5 us The input signal is sampled on the second rising edge of SCLK following the CS falling edge The user should ensure that 5 us elapse between the first falling edge of SCLK after the falling edge of THE PART POWERS UP FROM SHUTDOWN ON SCLK FALLING EDGE AS PM1 1 AND PMO 0 tio Sus 4 LEADING ZEROES CONVERSION RESULT DATA IN PM1 1 AND PMO 0 TO KEEP THE PART IN THIS MODE Figure 14a Autoshutdown Operation REV 0 11 AD7888 THE PART ENTERS SHUTDOWN AT THE END OF CONVERSION AS PM1 1 AND PMO 0 CONTROL REGISTER DATA IS LOADED ON THE FIRST 8 CLOCKS PM1 1 AND PMO 0 THE PART BEGINS TO POWER UP FROM SHUTDOWN 4 LEADING ZEROES CONVERSION RESULT PM1 AND PMO 0 TO PLACE THE PART IN NORMAL MODE DATA IN THE PART ENTERS SHUTDOWN AT THE END OF CONVERSION AS PM1
20. e internal reference is disabled SW1 in Figure 11 will open and the input impedance seen at the REF IN REF OUT pin is the input impedance of the reference buffer which is in the region of giga Q When the reference is enabled the input impedance seen at the pin is typically 5 kQ REF IN REF OUT O Figure 11 On Chip Reference Circuitry Table III Power Management Options PM1 PMO Mode 0 0 Normal Operation In this mode the AD7888 remains in full power mode regardless of the status of any of the logic inputs This mode allows the fastest possible throughput rate from the AD7888 Full Shutdown In this mode the AD7888 is in full shutdown mode with all circuitry on the AD7888 including the on chip reference en tering its power down mode The AD7888 retains the information in the control Register bits while in full shutdown The part remains in full shutdown until these bits are changed Autoshutdown In this mode the AD7888 automatically enters full shutdown mode at the end of each conversion Wake up time from full shutdown is 5 us and the user should ensure that 5 us have elapsed before attempting to perform a valid conversion on the part in this mode Autostandby In this standby mode portions of the AD7888 are powered down but the on chip reference voltage remains powered up The REF bit should be 0 to ensure the on chip refer ence is enabled This mode is similar to auto shutdown but allows t
21. going low is a falling edge this edge clocks out the first leading zero Thus the first rising clock edge on the SCLK clock has the first lead ing zero provided In applications where the first serial clock edge following CS going low is a rising edge the first leading zero may not be set up in time for the processor to read it cor rectly However subsequent bits are clocked out on the falling edge of SCLK so they are provided to the processor on the following rising edge Thus the second leading zero is clocked out on the falling edge subsequent to the first rising edge The final bit in the data transfer is valid on the 16th rising edge having being clocked out on the previous falling edge gt t e 4 LEADING ZEROS sje tconvert Figure 16 Serial Interface Timing Diagram REV 0 13 AD7888 MICROPROCESSOR INTERFACING The serial interface on the AD7888 allows the part to be directly connected to a range of many different microprocessors This section explains how to interface the AD7888 with some of the more common microcontroller and DSP serial interface protocols AD7888 to TMS320C5x The serial interface on the TMS320C5x uses a continuous serial clock and frame synchronization signals to synchronize the data transfer operations with peripheral devices like the AD7888 The CS input allows easy interfacing with an inverter between the serial clock of the TMS320C5x and the AD7888 being the only glue
22. he connection diagram in Figure 19 shows how the AD7888 can be connected to the SSI Synchronous Serial Interface of the DSP56xxx family of DSPs from Motorola The SSI is oper ated in synchronous mode SYN bit in CRB 1 with inter nally generated 1 bit clock period frame sync for both TX and RX bits FSL1 1 and FSLO 0 in CRB Set the word length to 16 by setting bits WL1 1 and WLO 0 in CRA An in verter is also necessary between the SCLK from the DSP56xxx and the SCLK pin of the AD7888 as shown in Figure 19 AD7888 DSP56xxx ADDITIONAL PINS OMITTED FOR CLARITY Figure 19 Interfacing to the DSP56xxx REV 0 AD7888 AD7888 to MC68HC11 The Serial Peripheral Interface SPI on the MC68HC11 is configured for Master Mode MSTR 1 Clock Polarity Bit CPOL 1 and the Clock Phase Bit CPHA 1 The SPI is configured by writing to the SPI Control Register SPCR see 68HC11 User Manual The serial transfer will take place as two 8 bit operations A connection diagram is shown in Figure 20 MC68HC11 O SCLK PD4 AD7888 ADDITIONAL PINS OMITTED FOR CLARITY Figure 20 Interfacing to the MC68HC11 AD7888 to 8051 It is possible to implement a serial interface using the data ports on the 8051 This allows a full duplex serial transfer to be implemented The technique involves bit banging an I O port e g P1 0 to generate a serial clock and using two other I O ports e g P1 1 and P1 2 to shift data i
23. he part to power up much faster POWER DOWN OPTIONS The AD7888 provides flexible power management to allow the user to achieve the best power performance for a given through put rate The power management options are selected by programming the power management bits i e PM1 and PMO in the control register Table III summarizes the options available When the power management bits are programmed for either of the auto power down modes the part will enter the power down mode on the 16th rising SCLK edge after the falling edge of CS The first falling SCLK edge after the CS falling edge will cause the part to power up again When the AD7888 is in full shutdown 10 the only way to fully power it up again is to reprogram the power management bits to PM1 PMO 0 i e normal mode In this case the device will power up on the 16th SCLK rising edge after the CS falling edge as this is when the power management bits become effective Power Up Times The AD7888 has an approximate 1 us power up time when powering up from standby or when using an external reference When Vpp is first connected the AD7888 will fully power up i e it powers up in normal mode If the part is put into shut down a subsequent power up will take approximately 5 us The AD7888 wake up time is very short in the autostandby mode so it is possible to wake up the part and carry out a valid conver sion in the same read write operation POWER VS THRO
24. ipation 450 mW Oja Thermal Impedance 124 9 C W SOIC R Gee SF PEA me side E E EEA 150 4 C W TSSOP Ojc Thermal Impedance 42 9 C W SOIC E E AERE shen 27 6 C W TSSOP Lead Temperature Soldering Vapor Phase 60 secs 2 2 0 2c eee ee eee 215 C Infrared 15 secs ccc eee eee 220 C ESD erosi reheat oetee tak EE S eee 1 5 kV NOTES Stresses above those listed under Absolute Maximum Ratings may cause perma nent damage to the device This is a stress rating only functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied Exposure to absolute maximum rating conditions for extended periods may affect device reliability Transient currents of up to 100 mA will not cause SCR latch up CAUTION ORDERING GUIDE Linearity Error Package Package Model LSB Descriptions Options AD7888AR 2 SOIC R 16A AD7888BR 1 SOIC R 16A AD7888ARU 2 TSSOP RU 16 AD7888BRU 1 TSSOP RU 16 EVAL AD7888CB EVAL CONTROL BOARD NOTES 1Linearity error here refers to integral linearity error Contact factory for availability 3This can be used as a stand alone evaluation board or in conjunction with the EVAL CONTROL BOARD for evaluation demonstration purposes This board is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in the CB
25. l sig nals up to half the sampling frequency fs 2 excluding dc The ratio is dependent on the number of quantization levels in the digitization process the more levels the smaller the quantiza tion noise The theoretical signal to noise distortion ratio for an ideal N bit converter with a sine wave input is given by Signal to Noise Distortion 6 02 N 1 76 dB Thus for a 12 bit converter this is 74 dB Total Harmonic Distortion Total harmonic distortion THD is the ratio of the rms sum of harmonics to the fundamental For the AD7888 it is defined as V2 V V2 V2 Vi TA THD dB log where V is the rms amplitude of the fundamental and V2 V3 V4 V and Vs are the rms amplitudes of the second through the sixth harmonics Peak Harmonic or Spurious Noise Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the ADC output spectrum up to fs 2 and excluding dc to the rms value of the fundamental Normally the value of this specification is deter mined by the largest harmonic in the spectrum but for ADCs where the harmonics are buried in the noise floor it will be a noise peak Intermodulation Distortion With inputs consisting of sine waves at two frequencies fa and fb any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa nfb where m n 0 1 2 3 etc Intermodulation distortion terms a
26. level of 1 6 V Mark Space ratio for the SCLK input is 40 60 to 60 40 gt Measured with the load circuit of Figure 1 and defined as the time required for the output to cross 0 8 V or 2 4 V with Vpp 5 V 10 and time for an output to cross 0 4 V or 2 0 V with Vpp 3 V 10 fts is derived from the measured time taken by the data outputs to change 0 5 V when loaded with the circuit of Figure 1 The measured number is then extrapolated back to remove the effects of charging or discharging the 50 pF capacitor This means that the time tg quoted in the timing characteristics is the true bus relinquish time of the part and is independent of the bus loading Specifications subject to change without notice 1 6V Figure 1 Load Circuit for Digital Output Timing Specifications REV 0 AD7888 PIN CONFIGURATIONS SOIC AND TSSOP 16 SCLK 15 DOUT 14 DIN TOP VIEW AIN1 5 Not to Scale 12 AIN8 AIN7 10 AING 9 AINS PIN FUNCTION DESCRIPTIONS Pin Pin No Mnemonic Function 1 CS Chip Select Active low logic input This input provides the dual function of initiating conversions on the AD7888 and also frames the serial data transfer 2 REF IN REF OUT Reference Input Output The on chip reference is available on this pin for use external to the AD7888 Alternatively the internal reference can be disabled and an external reference applied to this input The voltage range for the external reference is from
27. logic required The serial port of the TMS320C5x is set up to operate in burst mode with internal CLKX TX serial clock and FSX TX frame sync The serial port control register SPC must have the following setup FO 0 FSM 1 MCM 1 and TXM 1 The connection diagram is shown in Figure 17 AD7888 TMS320C5x ADDITIONAL PINS OMITTED FOR CLARITY Figure 17 Interfacing to the TMS320C5x AD7888 to ADSP 21xx The ADSP 21xx family of DSPs are interfaced to the AD7888 with an inverter between the serial clock of the ADSP 21xx and the AD7888 This is the only glue logic required The SPORT control register should be set up as follows TFSW RFSW 1 Alternate Framing INVRES INVTFS 1 Active Low Frame Signal DTYPE 00 Right Justify Data SLEN 1111 16 Bit Data Words ISCLK 1 Internal Serial Clock TFSR RFSR 1 Frame Every Word IRFS 0 ITFS 1 The connection diagram is shown in Figure 18 The ADSP 21xx has the TFS and RFS of the SPORT tied together with TFS set as an output and RFS set as in input The DSP oper ated in Alternate Framing Mode and the SPORT Control Reg ister is set up as described The frame synchronization signal generated on the TFS is tied to CS and as with all signal pro cessing applications equidistant sampling is necessary How ever in this example the timer interrupt is used to control the sampling rate of the ADC and under certain conditions equi distant sampling may not be
28. n and out see Figure 21 AD7888 ADDITIONAL PINS OMITTED FOR CLARITY Figure 21 Interfacing to the 8051 Using I O Ports AD7888 to PIC16C6x 7x The PIC16C6x Synchronous Serial Port SSP is configured as an SPI Master with the Clock Polarity Bit 1 This is done by writing to the Synchronous Serial Port Control Register SSPCON See user PIC16 17 Microcontroller User Manual Figure 22 shows the hardware connections needed to interface to the PIC16 17 In this example I O port RA1 is being used to pulse CS This microcontroller only transfers eight bits of data during each serial transfer operation Therefore two consecutive read write operations are needed PIC16C6x 7x SCK RC3 AD7888 ADDITIONAL PINS OMITTED FOR CLARITY Figure 22 Interfacing to the PIC16C6x 17x REV 0 APPLICATION HINTS Grounding and Layout The AD7888 has very good immunity to noise on the power supplies as can be seen by the PSRR vs Frequency graph However care should still be taken with regard to grounding and layout The printed circuit board that houses the AD7888 should be designed so the analog and digital sections are separated and confined to certain areas of the board This facilitates the use of ground planes that can be easily separated A minimum etch technique is generally best for ground planes as it gives the best shielding Digital and analog ground planes should be joined in only one place Both AGND pins of the AD78
29. ource impedances 65 THD vs FREQUENCY FOR DIFFERENT SOURCE IMPEDANCES 70 Vpp 5V 5V EXT REFERENCE Rin 1kQ Ciy 100pF g 75 1 Rin 509 Cyy 2 2nF a T 80 85 Riy 109 Ciy 10nF 90 0 15 10 89 21 14 31 59 42 14 49 86 INPUT FREQUENCY kHz Figure 10 THD vs Analog Input Frequency Analog Input Selection On power up the default AIN selection is AIN1 When return ing to normal operation from power down the AIN selected will be the same one that was selected prior to power down being initiated Table II below shows the multiplexer address corresponding to each analog input from AIN1 to AIN8 for the AD7888 Table II Channel Configurations ADD2 ADD1 ADDO Analog Input Channel 0 0 0 AIN1 0 0 1 AIN2 0 1 0 AIN3 0 1 1 AIN4 1 0 0 AIN5 1 0 1 AIN6 1 1 0 AIN7 1 1 1 AIN8 On Chip Reference The AD7888 has an on chip 2 5 V reference This reference can be enabled or disabled by clearing or setting the REF bit in the Control Register respectively If the on chip reference is to be used externally in a system it must be buffered before it is ap plied elsewhere If an external reference is applied to the device the internal reference is automatically overdriven However in AD7888 order to obtain optimum performance from the device it is advised to disable the internal reference by setting the REF bit in the Control Register when an external reference is applied When th
30. re those for which neither m nor n is equal to zero For example the second order terms include fa fb and fa fb while the third order terms include 2fa fb 2fa fb fa 2fb and fa 2fb The AD7888 is tested using the CCIF standard where two input frequencies near the top end of the input bandwidth are used In this case the second order terms are usually distanced in frequency from the original sine waves while the third order terms are usually at a frequency close to the input frequencies As a result the second and third order terms are specified sepa rately The calculation of the intermodulation distortion is as per the THD specification where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in dBs Channel to Channel Isolation Channel to channel isolation is a measure of the level of crosstalk between channels It is measured by applying a full scale 25 kHz sine wave signal to all nonselected input channels and determin ing how much that signal is attenuated in the selected channel The figure given is the worst case across all four or eight chan nels for the AD7888 PSR Power Supply Rejection Variations in power supply will affect the full scale transition but not the converter s linearity Power supply rejection is the maximum change in the full scale transition point due to a change in power supply voltage from the
31. s REV 0 5 AD7888 TERMINOLOGY Integral Nonlinearity This is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function The end points of the transfer function are zero scale a point 1 2 LSB below the first code transition and full scale a point 1 2 LSB above the last code transition Differential Nonlinearity This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC Offset Error This is the deviation of the first code transition 00 000 to 00 001 from the ideal i e AGND 0 5 LSB Offset Error Match This is the difference in offset error between any two channels Gain Error This is the deviation of the last code transition 111 110 to 111 111 from the ideal i e Vger 1 5 LSB after the offset error has been adjusted out Gain Error Match This is the difference in gain error between any two channels Track Hold Acquisition Time The track hold amplifier returns into track mode at the end of conversion Track Hold acquisition time is the time required for the output of the track hold amplifier to reach its final value within 1 2 LSB after the end of conversion Signal to Noise Distortion Ratio This is the measured ratio of signal to noise distortion at the output of the A D converter The signal is the rms amplitude of the fundamental Noise is the sum of all nonfundamenta
32. ss to this reference Alter natively this pin can be overdriven to provide an external refer ence voltage for the AD7888 The voltage range for this external 4 Analog input range from 0 V to Vpgr Vpp reference is from 1 2 V to Vpp 5 Versatile serial I O port SPI QSPI MICROWIRE DSP CMOS construction ensures low power dissipation of typically Compatible 2 mW for normal operation and 3 uW in power down mode The part is available in a 16 lead narrow body small outline SOIC and a 16 lead thin shrink small outline TSSOP package 3 Flexible power management options including automatic power down after conversion SPI and QSPI are trademarks of Motorola Inc MICROWIRE is a trademark of National Semiconductor Corporation REV 0 Information furnished by Analog Devices is believed to be accurate and reliable However no responsibility is assumed by Analog Devices for its use nor for any infringements of patents or other rights of third parties One Technology Way P O Box 9106 Norwood MA 02062 9106 U S A which may result from its use No license is granted by implication or Tel 781 329 4700 World Wide Web Site http www analog com otherwise under any patent or patent rights of Analog Devices Fax 781 326 8703 Analog Devices Inc 1999 AD 71888 SP E C FI CATI 0 NS Voo 2 7 V to 5 25 V REFIN REFOUT 2 5 V External Internal Reference unless otherwise noted fsc 2 MHZ Vip 2 7 V to 5 25 V Ta Tm to Tma
33. tion and the sampling capacitor acquires the signal on AIN CHARGE REDISTRIBUTION DAC CONTROL LOGIC SAMPLING A CAPACITOR SW1 0B it ACQUISITION AGND PHASE REF IN REF OUT 2 Figure 5 ADC Acquisition Phase When the ADC starts a conversion see Figure 6 SW2 will open and SW1 will move to Position B causing the comparator to become unbalanced The control logic and the charge redis tribution DAC are used to add and subtract fixed amounts of charge from the sampling capacitor to bring the comparator back into a balanced condition When the comparator is rebal anced the conversion is complete The control logic generates the ADC output code Figure 7 shows the ADC transfer function CHARGE REDISTRIBUTION DAC SAMPLING ia CAPACITOR vINO O CONTROL swt Ya LOGIC CONVERSION RER PHASE REF IN REF OUT 2 Figure 6 ADC Conversion Phase ADC TRANSFER FUNCTION The output coding of the AD7888 is straight binary The de signed code transitions occur at successive integer LSB values i e 1 LSB 2 LSBs etc The LSB size is Vgep 4096 The ideal transfer characteristic for the AD7888 is shown in Figure 7 below A 111 111 4 111 110 4 ea w 4 a Q 111 000 5 g 3 1LSB Vpep 4096 lt 011 111 4 he 000 010 5 000 001 000 000 2 T 0 5LSB Vper 1 5LSB ov ANALOG INPUT Figure 7 Transfer Characteristic REV 0 AD7888 TYPICAL CONNECTION DI
34. ughput rate for automatic shutdown with both 5 V and 3 V supplies POWER mW 0 10 20 30 40 50 THROUGHPUT kSPS Figure 12 Power vs Throughput REV 0 AD7888 THE PART REMAINS POWERED UP AT ALL TIMES AS PM1 AND PMO 0 cs f 1 SCLK u 16 u DOUT 4 LEADING ZEROES CONVERSION RESULT DIN DATA IN CONTROL REGISTER DATA IS LOADED ON THE FIRST 8 CLOCKS PM1 AND PMO 0 TO KEEP THE PART IN THIS MODE Figure 13 Normal Mode Operation MODES OF OPERATION The AD7888 has a number of different modes of operation These are designed to provide flexible power management options These options can be chosen to optimize the power dissipation throughput rate ratio for differing application re quirements The modes of operation are controlled by the PM1 and PMO bits of the Control Register as outlined previously Normal Mode PM1 0 PMO 0 This mode is intended for fastest throughput rate performance as the user does not have to worry about any power up times with the AD7888 remaining fully powered all the time Figure 13 shows the general diagram of the operation of the AD7888 in this mode The data presented to the AD7888 on the DIN line during the first eight clock cycles of the data transfer are loaded to the Control Register The part will remain powered up at the end of the conversion as long as PM1 and PMO were set to zero in
35. using an externally applied reference source See On Chip Reference section 1 0 PM1 PMO Power Management Bits These two bits decode the mode of operation of the AD7888 as shown in Table III PERFORMANCE CURVES Figure 2 shows a typical FFT plot for the AD7888 at 100 kHz Figure 3 shows a typical plot for the SNR vs frequency for a sample rate and 10 kHz input frequency 5 V supply and with a 5 V external reference 10 4096 POINT FFT SAMPLING Vpp 5V 5V EXT REFERENCE 100kSPS fin 10kHz SNR 70dB 72 5 S 50 S 72 0 b z no 70 71 5 90 i 110 i torch Ma ta i ld pii 71 0 0 12 21 24 41 36 62 48 83 0 10 89 21 14 31 59 42 14 FREQUENCY kHz INPUT FREQUENCY kHz Figure 2 Dynamic Performance Figure 3 SNR vs Input Frequency REV 0 7 AD7888 Figure 4 shows the typical power supply rejection ratio vs frequency for the part The power supply rejection ratio is de fined as the ratio of the power in the ADC output at frequency f to the power of a full scale sine wave applied to the ADC of frequency fs PSRR dB 10 log Pf Pfs Pf Power at frequency fin ADC output Pfs power at fre quency fs in ADC full scale input Here a 100 mV peak to peak sine wave is coupled onto the Vpp supply Both the 2 7 V and 5 5 V supply performances are shown Vpp 5 5V 2 7V 100mV p p SINE WAVE ON Vpp REF yy 2 488V EXT REFERENCE PSRR dB
36. x unless otherwise noted Parameter A Version B Version Units Test Conditions Comments DYNAMIC PERFORMANCE Signal to Noise Distortion Ratio gt SNR 71 71 dB typ fmn 10 kHz Sine Wave fsampre 125 kSPS Total Harmonic Distortion THD 80 80 dB typ fn 10 kHz Sine Wave fsampre 125 kSPS Peak Harmonic or Spurious Noise 80 80 dB typ fn 10 kHz Sine Wave fsampre 125 kSPS Intermodulation Distortion IMD Second Order Terms 78 78 dB typ fa 9 983 kHz fb 10 05 kHz fsampre 125 kSPS Third Order Terms 78 78 dB typ fa 9 983 kHz fb 10 05 kHz fsampre 125 kSPS Channel to Channel Isolation 80 80 dB typ f n 25 kHz Full Power Bandwidth 2 5 2 5 MHz typ 3 dB DC ACCURACY Any Channel Resolution 12 12 Bits Integral Nonlinearity 2 1 LSB max Differential Nonlinearity 2 1 LSB max Guaranteed No Missed Codes to 11 Bits A Grade Guaranteed No Missed Codes to 12 Bits B Grade Offset Error 6 6 LSB max Vpp 4 75 V to 5 25 V Typically 3 LSB 45 45 LSB max Vpop 2 7 V to 3 6 V Typically 2 LSB Offset Error Match 2 2 LSB typ Gain Error 2 2 LSB max Typically 30 LSB with Internal Reference Gain Error Match 2 2 LSB max ANALOG INPUT Input Voltage Ranges 0 to VREF 0 to VREF Volts Leakage Current 1 1 uA max Input Capacitance 38 38 pF typ When in Track 4 4 pF typ When in Hold REFERENCE INPUT OUTPUT REFIN Input Voltage Range 2 5 Vpp 2 5 Vpp V min max Functional from 1 2 V

ANALOG DEVICES AD7888 +2.7 V to +5.25 V Micropower 8-Channel 125 kSPS 12-Bit ADC in 16-Lead TSSOP handbook

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