Opamp isolation resistor calculations for a capacitive load

by user1850479   Last Updated July 11, 2019 22:25 PM

I want to add a capacitive filter for pulse shaping to the output of a transimpedance amplifier using an OPA847. Without the filter, the amplifier works great. With the filter, I get some oscillation because the filter presents a ~40pF capacitance to the TIA output.

The OPA847 datasheet recommends using a series isolation resistor to separate the capacitive load from the feedback circuit:

Driving capacitive loads

They give recommended values for a 20 V/V gain, and further state:

The criterion for setting the RS resistor is a maximum bandwidth, flat frequency response at the load. For the OPA847 operating in a gain of +20, the frequency response at the output pin is very flat to begin with, allowing relatively small values of RS to be used for low capacitive loads. As the signal gain is increased, the unloaded phase margin also increases. Driving capacitive loads at higher gains requires lower RS values than those shown for a gain of +20.

This is indeed the case, if I use the recommended 30 ohms (for 20V/V) in my transimpedance amplifier, the oscillation is hardly damped. I could just keep trying larger values until it works, but I'd like to understand how that value of 30 ohms is calculated, and how it would be calculated in the case of transimpedance amplifier which has transimpedance gain rather than voltage gain. This is probably obvious, but I'm missing something.

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