For a new design I am considering whether to use a Zero Drift Amp in a 2-pole Low Pass Sallen-Key analog filter. The cut-off frequency is low, 100Hz. Are ZDAs suitable for use in analog filters? What are the pitfalls I should be aware of?
I think there shouldn’t be any problems using a ZDA in a filter, as long as the resistances used in the filter are moderate (below Megaohms) and the rolloff of the filter is low enough in frequency (2-3 decades below chop frequency) that the clock spikes are well attenuated.
The higher the chop frequency of the zero drift amplifier, the higher attenuation of the clock spikes and noise bump around that frequency. For a 100Hz rolloff filter, something with a 10k-100kHz chopper clock like LTC20572058 (100kHz chop frequency) should be good enough.
Here's a sample design courtesy my colleague and filter genius, Philip Karantzalis. Apologies for it being a screenshot; we aren't allowed to upload .asc sim-able LTspice files.
Thanks a million Catherine. The LTC2057 was exactly the part I had in mind.
You could also look at the ADA4528-1 or ADA4522-1 depending on the supply voltages available.
You didn't say what your lowest signal level will be. If you have +/-15V available, and have DC to 100 Hz,
the venerable OP27 bipolar might be better. It is 3.8 nV/rt-Hz flatband with a 1/f corner at 2.7 Hz.
I figured out a workaround for the .asc restriction. Here's a .zip file with the .asc inside.
Zero Drift 100Hz LPF asc.zip
A colleague of mine (Glen B.) mentioned that 2057 does have somewhat high Ib, so 10x lower R's (and correspondingly scaled-up 10x C's) might be necessary if you see a lot of DC input offset. I think the input cap C1 will help lower the Ib somewhat, since it helps average the charge spikes from chopper switching, but it may not get rid of the problem entirely.