I am designing a voltage to current converter with the following specs: Supply Voltage: 5V dc, Input: 0V to 5V dc, Output: 0uA to 5uA dc.
I am using the AD8237 in-amp and the ad8538 as the first stage op-amp as shown in the circuit below
My load is represented by R6 in the circuit above but the real load can be modelled as a variable resistor from 10k to about 50MOhm. The current drop across the 100k R1 resistor is the same current that is going through my load and for a 1V input, I expect it to be 1uA. The U3 op amp is a unity gain buffer so that I can reliably measure the voltage across R6.The simulation in LTSpice is working as expected as shown by the nodal voltages shown in the circuit. However, the boards I have are giving me different values.
For a 1V input voltage, I am getting the voltage drop across R1 as 1V and the voltage drop across R6 also as 1V when I am expecting the voltage drop across each resistor to be 100mV to give me a 1uA current as show in the simulation. The measured AD8237 output is ~2.8V. I have tried different op-amps with the AD8237 for my setup and I am still not getting expected results. However, a previous version of my board used a TI INA326 as the instrumentation amplifier with a similar setup and it was giving me expected results.
Something is going wrong with the boards I have in the feedback circuit which is raising the output of the AD8538 and I am not sure what it is. I am hoping someone can help me track down the source of this issue.