Cure is isolation by resistors, try to insert R in between cap & input, and inverting input is no exception
Thanks, this was a good clue.
The voltage dependence of the input bias looks really bad.
I agree, when I saw this, I also thought the same as you - there is still value in bootstrapping an AZ amplifier to make the input bias compensation (current injection through high value resistor) easier. Do you have links to the microchip application notes?
How does this issue affects the performance actually? How to improve it?
You have to build the circuit and measure it. No amount of simulation or discussion will help here. (this is coming from someone who really likes simulation!)
I am working toward a test setup for characterizing low frequency noise. There are now some outstanding precision JFET parts from TI, so they would have to perform very well indeed to be worth using.
I eagerly await your results. Do you mean OPA140 and friends? (or JFE2140?)
Some years ago somebody explained about the spikes from the opamp inputs generated by the chopper. Since then i started to place a capacitor to Gnd also on the inverting input.
This is the same thing I had in mind, and I've seen it on datasheets too. However, as the following will show... this does *not* apply to OPA189.
Continuation of experimentationWith all those comments in mind, in an attempt to tame the OPA189, I explored various topologies involving R and C and C on the inverting input too. All of them failed except one - simple resistive-only isolation of both OPA189 inputs. It hates any kind of additional capacitance on the inputs (which defies the 'taming charge injection spikes' hint we've all seen before).
See attached image.
I think the only reliable rules so far are:
- You have to measure reality. Discussion or simulation no good here.
- Once you've picked & measured an AZ amplifier for your design, you cannot substitute it for another part unless you perform all the same testing again.