Having oscillation problems with an AD8304 setup to read the monitor current output from an APD voltage generator. Adding a few hundred ohms of series resistance helps eliminate an oscillation of ~1Mhz in the current range of 10uA to 100uA, but still have oscillation at ~40kHz for current <10uA. Currently setup per the datasheet with 750ohm and 1nF in series on input. I suspect I need to fine tune this and the driving impedance (or cover it up with series resistance). Looking for some guidance on how to go about this. In other words, try to keep zero at about the same frequency as the 750/1nf combination, maintain a certain resistance seen looking out from Input pin, etc..

Hi joelp,

I apologize for the belated response. ADI was on a holiday shutdown for the majority of the last 2 weeks.

Do you see an oscillation on the VLOG/VOUT output? And/or does the output response of the log amp, which is supposed to be a linear-in-dB response, go non-linear in the certain current ranges mentioned above?

Unfortunately, there is no closed form equation to generate the input RC compensation network. The 750 Ohms and 1nF was a empirically found solution, that provided stability for the log amp but did not slow down the response time/bandwidth too much.

Do you need the full bandwidth of the input? If you don't, I would try increasing the 1nF cap. I would try 10nF and 100nF with the ~1k resistor.

Thanks,

Joel