OPA197: Feedback Resistor Selection

Hey Zachary, 

The selection of the Feedback R (and hence gain R) gets into several concerns - 

1. noise

2. phase margin as there is an added loop pole back to the V- node cap

Could you include your TINA file, attaching it using the the little paperclip symbol across the top here, 

Hi Michael,

Attached is the simulation for this circuit.

V/R,

Zach

OPA549 Test Circuit with Diff Amp.TSC

Hi Zach,

It is unnecessary to place 1Mohm resistor in OPA197 circuit. I changed from 1MOhm to 100kOhm, and you may still able to lower slightly. You will increase the unwanted thermal noises of the op amp circuit, see the attached plot.  

I think that R31 = 100kOhm is your floating ground simulation? 

Best,

Raymond

Hi Zachary,

why do you want to use 1M feedback resistors?

Kai

Hey Zachary, thanks for the TINA file, makes it easy to check noise, a quick way to expose instability - yep the spike is at the output of the first stage, those 1Mohm R's are creating a low pole in the feedback network with the input C, 

Here I changed those to 10kohm and reran, that took care of it,