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Unexpected output in a Voltage to Current Converter using AD8237 and AD8538

Category: Hardware
Product Number: AD8237


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.

Thank you,


Parents Reply
  • The unexpected output perhaps be due to the internal configuration of AD8237 vs INA326. AD8237 uses indirect current feedback wherein it checks the output of the in-amp then fed back to the FB pin of AD8237. Whereas the INA326, is like a 3-opamp in-amp that the output is like a buffer and internally the gain is compensated. You can try other ADI inamps those using same config as INA8236 such as LT1167 or AD8421.

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