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ADA4522-4 apparent crosstalk and short-circuit input

Hello, 

I'm using an ADA4522-4 as a buffer and amplifier for a simple current sense circuit and I observe a strange behavior where one inputs starts to sink a lot of current when another amplifier saturates. 

My circuit consists of a 330kΩ sense resistor intended to make a voltage from 0 to 3.3V when passing a current from 0 to 10 µA from V_fiber_-.
This voltage (call it I_leak_raw) is then buffered by two Opamps in the ADA4522-4 and sent to an ADC for sampling. Let's call them Channel C and D.
See schematic sections.


Channel C: One of the Opams is configured as a simple buffer (gain of 1) where the I_leak_raw is connected to positive input and the output is connected to the negative input, and to one ADC channel.
Channel D: The other Opamp is configured as a non-inverting amplifier with a gain of 1000X, and the output is sent to another ADC channel (and limited by a 3.6V Zener diode). The intention here is to be able to measure nA range currents with this channel.

As the I_leak_raw grows, channel D reports very high sensitivity and then saturates, and then channel C should continue to report the rest of the range. Surprisingly, it seems that when channel D saturates, the positive input into channel C starts sinking considerable current (almost acting like a short to ground) and therefore negates the impedance of the 330 kΩ sense resistor. 

Channels A and B  are used as voltage buffers for other signals and don't seem to be affected, but thorough testing is pending, so perhaps they are also sinking current when Chanel D saturates. 

Is this behavior expected? Or am I abusing the Opamps with this configuration? 
Can someone recommend changes to the circuit (or pointers to similar) to facilitate this dual range (dual gain) setup?
I have tried adding a 1 kΩ resistor on the input to channel D but this did not seem to have any effect. 

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  • I figured out the issue. 
    The OpAmp starts sinking considerable current when its output exceeds ~4.97 V.  This is reproducible in simulation. This max output voltage is listed on the spec sheet. What is not listed is that the device will sink current. 
    My workaround has been to place a large (1 MΩ) resistor on the input to decouple the current sink of channel D and prevent the output from saturating much beyond the limit.   This works in simulation and will soon be fabricated and tested. 
    Thank you everyone for the input so far!

Reply
  • I figured out the issue. 
    The OpAmp starts sinking considerable current when its output exceeds ~4.97 V.  This is reproducible in simulation. This max output voltage is listed on the spec sheet. What is not listed is that the device will sink current. 
    My workaround has been to place a large (1 MΩ) resistor on the input to decouple the current sink of channel D and prevent the output from saturating much beyond the limit.   This works in simulation and will soon be fabricated and tested. 
    Thank you everyone for the input so far!

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