Stability of a dual opamp integrator - Page 1
Forum section: electronics / projects

author: Sambeet
author_rank: Newbie
timestamp: April 15, 2022, 05:31:20 am
content: I found the attached circuit on the internet. I wanted to use it to integrate the output of a Rogowski coil. However, when we analyse this circuit, I find that it has zero phase margin. There is also substantial gain peaking. My question is how can we use this circuit practically if it is so close to instability. I have attached the image of the circuit and the image of the MATLAB simulation.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2022, 06:35:06 am
content: The circuit should be resonable stable. The problem may be in how the phase margin is calculated: the integrator has a 90 deg phase shift as the normal, nominal case and this a 180 deg. phase from in to out at the upper end is not a problem. For a stability analysis one has to calculate the phase for the loop gain, not the overall transfer function.I have tested essentially the same circuit (OPA172/TLE2081 instead of the OPA1641, but these are the same speed) and it was stable. There can still be some ringing with square wave excitation and this can be improved a little with an series RC (e.g. 100 to 220 ohms and some 200-500 pF) from the integrator input to ground.The combination with the relatively slow OPA145 could even be tuned to be a bit more aggressive with less divider between the stages. The divider ratio 1:6 is about the limit for 2 OPs of the same speed.

author: Sambeet
author_rank: Newbie
timestamp: April 15, 2022, 07:30:50 am
content: Thank you for your answer. I verified the circuits again, this time using LTSPICE as mentionedhttps://www.analog.com/en/education/education-library/videos/5579254320001.html. I am attaching the images. I noticed something that if we tie the inverting terminal of OPA1641 to the ground(i.e. first scenario) we get a system near instability, however, if we tie the inverting terminal of OPA1641 to the feedback path(i.e. second scenario), the circuit becomes stable with a near about 90 deg of phase margin. The two scenarios are in the attached screenshot.I was of the opinion that both scenarios are identical considering the OPA1641 being tied to the zero references, the actual ground reference in the first scenario and the virtual ground in the second scenario. Why are they not identical with respect to stability?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2022, 08:25:18 am
content: The OPA1641 inverting input has to by tied to the integrator node, otherwise the circuit is likely unstable. So having the inverting input to ground is never a real option, as it would add the full open loop gain to the loop.The OPA1641 with the capacitor works as a kind of inner loop, doing the main integration. The OPA145 is more like a small correction forcing the virtual ground to be even closer to ground. The loop of the OPA145 needs the integrator to do it's job and have the 2 input effectively linked fast enough. This is why the OPA145 together with the divider needs to be slower than the OPA1641.

author: magic
author_rank: Super Contributor
timestamp: April 15, 2022, 03:58:15 pm
content: Feedback through C1 renders U2 a unity gain follower at frequencies above the RC corner of C1 and RN1. This is safe to put in the feedback loop of U1 as long as it has negligible closed loop phase delay over the bandwidth of U1. With R10 and R11, the necessary bandwidth is further divided by their attenuation ratio.Without feedback, U2 is a very high gain amplifier with 90° (or more) phase lag over all of U1 bandwidth, except DC and very low frequencies. Opamps don't like gain in their feedback path and don't like that sort of phase delays, this is a guaranteed oscillator.