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OPA857EVM-978: Issue setting OPA857EVM-978 into test mode

Joe G
Prodigy 30 points
Part Number: OPA857EVM-978
Other Parts Discussed in Thread: OPA857, OPA857EVM

Tool/software:

Hello,

I recently purchased an OPA857EVM-978 evaluation module, along with some bare OPA857s for testing.

After unpacking the board, I wanted to use it in test mode following the instructions in the user guide, but I can’t get it to work. The output remains at 0 V, and I’m not sure if I’m missing a step.

Here’s my current setup:

  • Vcc: 3.3 V

  • Test_In: 2.3 V DC bias

  • Test_SD: 3.3 V (jumper set)

I have a few questions:

  1. Should jumper J4 be left in place, or removed when using test mode?

  2. Is there anything else I might be missing to get the output active?

Thanks Slight smile

  • 0
    TI__Mastermind 29345 points

    Hello Joe,

      Thanks for the details on your setup!

      For J4, you will need to remove the jumper for test mode. In normal operation you would place J4 for connection to VS+. 

      I believe the issue is with Test_IN pin. I find the OPA857 datasheet easier to read:

       On the EVM there isn't a way to add an AC coupling capacitor, instead would you be able to set your DC bias to around 2.1V? Let us know if the DC change fixed the issue, you should see that OUT and OUTN is equal to around 1.8V. Then you will be able to add an AC voltage on this DC bias. When I tested it on the OPA857EVM, I believe I used around 70mVpp sine wave on top of 2.1V DC to achieve 500mVpp output (TIA output, will be attenuated after loading) at 20kOhm gain selection. And 280mVpp AC signal on 2.1V DC bias for 5kOhm gain selection. 

    Thank you,
    Sima