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OPA858: Gain Switching for OPA858

Part Number: OPA858
Other Parts Discussed in Thread: TMUX1112, OPA859, OPA857, OPA3S2859, TMUX1511

Hello everybody,

at the moment we are busy designing an application to evalute OTDR Pulses. So we choose OPA858 as a fast Opamp in a TIA application.

Application itself is working but we run into trouble consindering gain switching. There is a need to cover very small pulse (few nA) up to several uA or even more to get a adequate picture of what is possibly happening on the fibre. Thats impossible to do with just one feedback resistor, that means we have the need to switch gain.

We thought of using TMUX1112 analog switches for that in combination with clamping diodes to optimze recovery time. In principle everything works as expected but unluckily there is permanent HF- ringing even whe no Photiodiode is connected. Tunning by feedback capacity reduces amplitude to a minimum but it is impossible to get rid off it completely. Feedback resitors are 1k, 20k and 100k. We know that having a bigger feedback loop because of the analog switch is not best choice for HF. But anyhow, is there any hint or recommendation when having a set up like this with OPA858? Or is it impossible to do that? Would migrating to OPA859 be an option?

I have already reda some of the other threads focusing with the multistage issue but up to now I could not find a sufficient answer.

Do you have any additional hint, tip or idea what to to best in that case?

Thanks in Advance.

Regards Christian