Electronics > Projects, Designs, and Technical Stuff

Opamp output question

(1/2) > >>

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

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.

That 0.05V/┬Ás 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? ;)

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.

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.


[0] Message Index

[#] Next page

There was an error while thanking
Go to full version