I have an application where capacitors for a backup supply are charged to about 2.6V through a diode from 3.3V. I want to be able to monitor the capacitor voltage using a circuit that draws very little current (<10nA) from the backup supply when the main 3.3V supply goes down. One idea I had is to use an op-amp follower (powered from the main 3.3V supply) to buffer the backup supply voltage (see conceptual diagram). However if the amplifier is not powered but still has 2.6V on the input then it could damage the amplifier or at least result in excessive input current.
I can't power the follower amplifier from the backup supply since it has to drive a voltage divider (and supply current would be too high even with no load).
I don't think this approach is feasible but wanted to ask in case ADI has amplifiers suited for this type of thing or if someone has an idea for a modified approach.
Thanks - Jason
After finding a couple of posts that address this question fairly well I am going to request that it be closed.
You are correct; most op amps would have trouble with this.
Look at the ADA4096-2.
Thanks. Yes, I found the ADA4096-2 and the ADA4177-2. The ADA4177-2 has lower input bias current but the packages are larger than the ADA4096-2 in LFCSP.
The OVP feature prevents damage in my example but I'm not sure whether operating within the OVP window still guarantees the operating specs. For example, if operating the ADA4096-2 at -V=0V, +V=0V and Vin=2.6V does the input bias current spec still apply?
The ADA4177 datasheet has a detailed description of the OVP circuitry and what happens as the input exceeds the supply rails (large input current) so I think this answers my question. I assume that the ADA4096 OVP works the same way.
Just catching up if you are already done with your design. Probably, you could also check our Over the Top amplifiers which might suit your requirements, such as the LT1782, LT6015 to name a few.