A Panda II I’m running with a shield that contains a a quad OP amp IC (LM324AN) running off the 5 volt Panda supply. When the Panda goes in and out of hibernation there is a small ~50 mv spike in the supply voltage. Unfortunately the amplifier as configured uses an voltage offset of ~1.6 volts that also sees the spike at the input and thus also gets amplified. The offset is needed to get a full 0-3.2 volt swing for the Panda A/D. After amoplification there are full scale voltage spikes that can present problems in analyzing the digitized signal (see pic).
I’ve tried adding 320 uf of filtering on the 5 volt rail, but with little effect. I can add another regulator to power the amp off the 12 volt supply and bypass the Panda power circuits, but don’t want to do that unless absolutely necessary. I suppose I could also try another amplifier circuit or forgo the offset by tying to ground and use only half the signal, but not an elegant solution either.
I doubt there’s a simple solution, but I thought this might be worth mentioning in case it helps someone else. On the other hand if someone has a idea I’m all ears (small Yoda joke).
I agree as it is really a problem with the amplifier and not the Panda. However, I may have tried the big capacitor in the wrong place. I rewired it to filter the 1.6 reference voltage that’s at the Op Amp input and the spike is much smaller - maybe tolerable. If it works well enough I’ll followup with a closing post.
By putting the filter on the offset voltage input at the first stage of the Op Amp instead of on the 5 volt supply, the spike was reduced to 100 mv at the output which should be tolerable. 220 uf was enough. It takes a few seconds to achieve offset at startup, but other than that I can’t think of any downside. Thanks for getting me thinking, Gus.
Actually Gus, your answer did help. It made me recognize that as long as that small glitch was on the Op Amp input, I was sunk. Realizing that,m I refocused on the input and offset voltage rather than the 5 volt supply. Your answer was just what was needed. Thanks.