With honor to Walt Jung (of ADI, et al), low values of resistors can cause detectable THERMAL distortion. Drive the opamp with 20Hz and 2,000Hz; use a spectrum analyzer, and you'll see the 2,000Hz output with some 20Hz sidebands.
Which means what? Use physically larger resistors. Or experiment with resistors having different resistive element structures? The very thin metal-file-spiral-trimmed has a very fast timeconstant for heating/cooling, as heat is dumped into the ceramic/clay core.
And the opamp may need a BUFFER, to avoid generating thermal distortion as the silicon undergoes transient heating, as the UP transistors turn off and the DOWN transistors turn off.
For example, here is circuit with 1nanoVolt/rtHz opamp, and 26 ohm resistors; the input is 100 microVolts PeakPeak; notice the very wide bandwidth (out past 10MHz) causes a poor Signal-Noise-Ratio. With such a small input, the Transient Thermal Distortion is only 1.9 nanoVolt. Wonder what happens for larger input voltages, using the same resistors?
Here is what happened for 10X larger input: 1,000 microVolts PeakPeak. The first two stage Resistors are unchanged; we get lots of thermal Distortion ( 110 microVolts). To avoid overdriving the ADC, the 3rd gain stage now only provides 20dB gain.
What are the resistor values?
Stage 1 (S1): Rg = 26 ohms, Rfb = 497 ohms (gain of 26 dB)
Stage 2 (S2): Rg = 19 ohms, Rfb = 282 ohms (gain of 24 dB)
Stage 3 (S3): Rg = 1046 ohms, Rfb = 9422 ohms (gain of 20dB)
Again, the thermal distortion with 100uVPP input was 1.9 nanoVolts.
Yet with 1,000uVPP, that distortion soared to 110 microVolts.
Separate topic: the adi opamp has only 70dB PSRR at 10KHz. 70dB is 3,000:1.
So what? Will a thermally-noise VDD-regulator be a problem? Some LDOs have internal equivalent Rnoise of 10,000,000 ohms (Often in poly-silicon servo-feedback resistors, and in diffpairs operating in subthreshold at 100nanoAmp
currents). This produces 1 microvolt per rootHertz random thermal noise on the "clean" VDD rail. Is this a risk?
If you have 60dB PSRR at 10KHz, that 1microVolts becomes 1 nanoVolt Referred to Input, which is a 3dB increase of the opamp's noise floor. And at 100KHz, the opamp has only 50dB PSRR (from a datasheet plot).
Summary: pay attention to the VDD rail random noise. And don't think about using switchRegs in these systems, unless you
---- use magnetic shielding
---- use electric-field shielding
---- pay attention to building "local batteries" for the opamp's two rails
---- design the Ground, with slits, etc to keep trash away from the opamp