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LINEAR Design Note 502 Manual

1. DESIGN NOTES Matched Resistor Networks for Precision Amplifier Applications Design Note 502 Tyler Hutchison Introduction Some ideal op amp configurations assume that the feedback resistors exhibit perfect matching In practice resistor non idealities can affect various circuit param eters such as common mode rejection ratio CMRR harmonic distortion and stability For instance as shown in Figure 1 a single ended amplifier configured to level shift a ground referenced signal toa common mode of 2 5V needs a good CMRR Assuming 34dB CMRRandno input signal this 2 5V level shifter exhibits an output offset of 50mV which can even overwhelm the LSB and offset errors of 12 bit ADCs and drivers Figure 1 A Single Ended Op Amp Used as a Level Shifter Foranopamp 34dBisaless than ideal CMRR However a feedback network of 1 tolerance resistors can limit the CMRR to 34dB regardless of the op amp s capabili ties Highly matched resistors such as those provided by the LT 5400 available in 0 01 0 025 and 0 05 matching ensure that the designer can approach or meet amplifier data sheet specifications This design note compares the LT5400 with thick film 0402 1 tolerance surface mount resistors CMRR harmonic distortion and stability are considered with these resis tors for feedback around an LTC6362 op amp as shown in Figure 2 Common Mode Rejection Ratio In orderto obtai
2. ften difficult to achieve since data sheet specifications assume ideal components Carefully matched resistor networks such as those supplied by the LT5400 enable precision matching orders of magnitude better than discrete components ensuring data sheet specifications are met for precision ICs Table 1 For a Given Power Level the LT5400 Behaves More Linearly Than Other Resistor Types SOURCE HD3 120 00 AT MAX POWER 12Vpms 56mW into 1kQ RESISTOR TYPE POWER RATING HD3 56mW POWER HD3 1 14th RATED POWER LT5400 0 8W 117dBc 117dBc 5 Through Hole 0 25W 100dBc 114dBc 1 Through Hole 0 25W 115dBc 119dBc 1206 Thick Film 0 25W 104dBc 115dBc 0805 Thick Film 0 125W 93dBc 117dBc 0603 Thick Film 0 1W 89dBc 117dBc 0402 Thick Film 0 068W 72dBc 104dBc Data Sheet Download www linearcom Linear Technology Corporation 1630 McCarthy Blvd Milpitas CA 95035 7417 408 432 1900 FAX 408 434 0507 e www linear com For applications help call 978 656 4700 Ext 3741 dn502f LT AP 0512 196K PRINTED IN THE USA TECHNOLOGY LINEAR TECHNOLOGY CORPORATION 2012
3. ividual segments and the ex posed pad Incontrast typical surface mount resistors without the tight layout typically exhibit significantly less parasitic capacitance The effect of inter resistor capacitance can be mitigated when the exposed pad is grounded However even after grounding the exposed pad this capacitance still EXPOSED PAD C EXPOSED C EXPOSED C EXPOSED C EXPOSED R R C INTER C INTER C INTER ees C INTER R R R R C EXPOSED C EXPOSED JE C EXPOSED T C EXPOSED F EXPOSED PAD Figure 3 A Simple Model of the Distributed Capacitance in a Matched Resistor IC The Sum of R Components Creates an Equivalent Single Resistor The Net Effect of C inter is 1 4pF and the Net Effect of C Exposen is 5 5pF DN502 F03 affects circuit stability by forming a parasitic pole onthe order of the total resistance times the total capacitance Since overshootis inversely proportional to phase mar gin minimizing step response overshootis a good way to ensure circuit stability The uncompensated LT5400 configuration exhibits 27 compared to 17 overshoot from the 0402 configuration However the compensa tion capacitor necessary to achieve 8 overshoot is approximately the same in both configurations 18pF with the LT5400 15pF with 0402 resistors With nearly identical compensation the two circuits display similar stability characteristics Conclusion The actual performance of precision amplifiers and ADCs is o
4. marks are the property of their respective owners 05 12 502 A bench test of the circuit yielded a CMRR of 50 7dB highly resistor matching limited with 1 tolerance resistors and 86 6dB with the LT5400 In this case a 2 5V common mode input would result in an offset of 1 5mV with 1 thick film resistors and an offset of 23uV with the LT5400 making it suitable for 18 bit ADC applications where DC accuracy is critical Harmonic Distortion Harmonic distortion is also important when choosing resistors for a precision application A large signal voltage across a resistor may significantly change the resistance depending on the size and material This problem occurs inanumber of chip based resistors and naturally becomes more severe as the power levels at the resistors increase Table 1 compares the distortion of thick film through hole and the LT5400 resistors based on high power drive and similar power drive The results suggest that for a given signal the LT5400 distorts the signal much less than other resistor types Stability Figure 3 shows a model of the distributed capacitance between resistors in the LT5400 To achieve high preci sion matching and tracking in the LT5400 many small SiCr resistors are configured in series and parallel As a result of the complex interdigitation the LT5400 resistors can be modeled as a Series of infinitesimal resistors with parasitic capacitance between adjacent segments and between ind
5. n precise measurements in the presence of common mode noise a high CMRR is important Input CMRR is defined as the ratio of differential gain Vout oiFF Vin pier to the input common mode to differential conversion Vour piFr Vin cm Figure 2 Fully Differential Op Amp Configured for Vour Vin 0 2 In ideal single ended and fully differential amplifiers only the input differential level affects the output volt age However in real circuits resistor mismatch limits the available CMRR Consider this circuit configured to attenuate a 10V signal toa 2V signal Using typical sur face mount resistors with 2 matching 1 tolerance the worst case CMRR contribution from the resistors is 30dB With 0 01 tolerance 0 02 matching the worst case CMRR contribution from the resistors is 70GB A limiting factor in the CMRR equation is t i 2 R2 R4 This expression reduces to the resistor matching ratio for typical resistors butthe LT5400 takes an additional step to offer improved CMRR by constraining the matching between resistor pairs R1 R2 and R4 R3 By defining this equation as matching for CMRR the LT5400 offers accuracy that s better than just the resistor matching ratio For example the LT5400A guarantees 2 R4JR1 lt 0 005 R CMRR which improves worst case CMRR to 82dB 4J LI LTC LTM Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation All other trade

LINEAR Design Note 502 Manual

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