I bumped into a very strange behavior of ADA4627 op amp, and will be very grateful if you give me any tiny insight into what's happening. Right now I even don't know where to look for solution.
Here is our scheme, as it appears right at the moment:
The goal is to measure currents of several photodiodes. PDs are unbiased, currents lie in the range from tens of nA to several mkA and look like pulses with fronts of 7-10 mks and duration 20-50 mks (we can tune the duration, actually, but try to lower it to increases the speed of measurements). The desired measuring frequency is 50-100 kHz and noise of the order of several nA. We took EVAL-AD76MUXEDZ board with AD7699 ADC. The board allready had input amplifiers ADA4841-1, but those had too large input leakage current and we replaced them with ADA4627.
At moderate currents all works well. The mystic things come when upper level of input current pulse drives the amplifier into saturation. The trailing edge of pulse at the amplifier output suddenly jumps about 20 mks later than it was at lower current level. It happens for at least 4 of our PDs. And it does not happen with the very same PDs when they are loaded on to AD8605 op amp. But ADA4627 is preferrable, since it permits larger dynamic range at the same level of noise (25 V output vs 5 V).
1. What the heck is going on? I don't know how to find any piece of info about this effect because I just don't know how to call it.
2. There are some components left from the initial board scheme that we don't really need - R2, R3 and C3. Can they be somehow involved?
3. We are going to increase the ADA4627 positive power supply to +24 V to achieve maximum dynamic range. Will that help with this effect? Will it at least shift the effect to higher currents?
4. Just when writing this post I realized we have only +5 V ADC supply. As its manual tells, AD7699 has protective diodes that open when input voltage exceeds supply rail. Can that be the reason?