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

Category: Hardware
Product Number: AD8237

Hi,

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,

Tari

Parents
  • Would you be able to isolate the AD8237 from the rest of the circuit  say do a simple Vos or gain measurement with input of 10mV or so. What is its output?

  • On an isolated circuit, the AD8237 is working as expected giving me a gain of 10. For calculating Vos, I used the circuit below on a breadboard and I measured 0.9mV, which is very close to the 1mV LTSpice gave me.

  • I suspect the unexpected output of your circuit has something to do with the stability of your system since the output of the AD8237 loops back to your AD8538. Try to see if it works if you put some compensation caps in your output. Hope this helps.

  • I tried putting either a 10nF or  a100nF across the output of the AD8237 and negative input of of the AD8538 and that did not stabilize the system. I also tried to connect the capacitor from the AD8237 output to ground and the results are still not what I would expect.

  • In this isolated circuit, would you be able to test this in actual same as the way you would expect the output from AD8538? say put an input of 100mV (not grounded input). also since you've isolated your AD8237, could you also check what is the actual output of your AD8538 without the inamp? what is the purpose of the R3?

  • A 100mV input (not grounded) in the isolated schematic still gives me a gain of 10 as expected. This circuit still includes C1, R1, R2, R3 and R6

    Removing the inamp  and connecting R3 to the output of the AD8538 gives me 1V as the output of the AD8538 if I use a 1V input. R3 was used to stabilize the circuit in the original design application document that I am used to create the circuit. I used 10k as it gave a clean result on the simulation instead of the 100k used in this application note

  • 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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  • 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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