Opamp output question - Page 1
Forum section: electronics / projects

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 12, 2021, 12:42:09 pm
content: Hello all, I have the following circuit implemented on a PCB, it is a basic non-inverting amplifier, with an amplification factor of about 55. There are some amendments: R340 has been changed to 180Ohm and R341 has been changed to 10k (to offset a little the effect of the bias input currents). The output drives the input of another opamp, through an additional 15k resistor (not shown in the diagram)The plot below shows the input voltage waveform (yellow) and the output voltage waveform (green). I do not understand why the output is not fully saturated in the marked area, the input voltage level multiplied by a factor of 55 is way below -5V... Any thoughts? BR, Cristian

author: Alti
author_rank: Frequent Contributor
timestamp: May 12, 2021, 01:55:20 pm
content: This is not a standard opamp. Looks like the "slew enhancement circuit" kicks in initially (160V/us) and then it turns off to allow slow slew rate to reach the final output level. Just a suspicion, I have no experience with such weird opamps.

author: magic
author_rank: Super Contributor
timestamp: May 12, 2021, 02:16:39 pm
content: That 0.05V/µs slew, one might hope for at least jellybean-level performance even without slew boost.Plus, this chip needs to run some reasonable bias on the input devices at all time to hit its noise target.Dunno, is shutdown well connected?

author: Alti
author_rank: Frequent Contributor
timestamp: May 12, 2021, 02:47:28 pm
content: Also, biasing.Low offset voltage - yes, but this can only work with similar impedances of nodes. Inverting has 177R and non-inverting ? undefined value. With 0.5uA bias this is 90uV, close to worst case 125uV offset.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 12, 2021, 05:10:56 pm
content: I suspect this may have something to do with the "Output Overdrive Recovery Time" behavior. The datasheet only gives this parameter for G=+2, but I expect this time to increase with the gain.To figure it out, try:- Lowering the gain, and see how it affects this;- Lowering the input amplitude, and likewise.

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 19, 2021, 06:25:31 am
content: Thank you all for your valuable insight. Today I had time to look again at this problem, scope in hand. It turned out it was something much simpler than that: error measurement due to uncompensated probeOnce I properly compensate the probe, I get an expected behavior.The saturated output is still not aperfectlyhorizontal line, but I suppose this is due to the output operating basicallyatthe negative rail, in an area where it is not exactly linear; the output voltage swing is specified up to 20mV above the negative rail.Best regards,Cristian

author: harerod
author_rank: Frequent Contributor
timestamp: May 19, 2021, 12:37:38 pm
content: brumbarchris, a heartfelt "thank you" for coming out with that feedback.I had looked at your question, even discussed it with my mentor and we couldn't come up with a solution worth a reply. Nothing beyond - RTFM and check your setup. A snarky "why do breadboard kids always use 1GHz GBW opamps" crossed my mind as well. Shame on me.A similar thing cost me two hours the other day: channel 3 of my TPS2024B shows some offset drift after several weeks. 200mV is no biggy when debugging bus timings, but it thoroughly ruined my afternoon when I checked a video buffer. One ACAL later - peachy.Edit: typo

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 19, 2021, 12:54:16 pm
content: No problem! Thing is: it is not the first time I get bitten by an uncompensated probe, I should've known better. The worst thing is that I firmly believe it will also not be the last time either...BR,Cristian

author: harerod
author_rank: Frequent Contributor
timestamp: May 19, 2021, 01:35:13 pm
content: Experience is the sum of all past mistakes. I dare call myself experienced. Not so occasionally, I even charge money for that...