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RTOS/OPA857EVM: TIDA-00725 Signal-Chain Circuit Schematic functional testing

Part Number: OPA857EVM
Other Parts Discussed in Thread: OPA857, TIDA-00725,

Tool/software: TI-RTOS


Is there anyway to test the functionality of signal chain circuit before connecting it to photodiode. I mean which signal should I feed to this board and what output should I observe at "OUT+" and "OUT-"  ?

  • Hello Ahsan,

    This is the function of the Test_In pin. To enable test mode, you will need to set Test_SD to +Vs by connecting jumper JP1. This Test_In pin is connected to an internal V-to-I converter so that you can provide an input voltage to test the circuit.

    As stated in the EVM user guide, Test_IN "can be driven through the SMA connector J1. In test mode, set the dc bias of the input signal to approximately 2.1 V. The output test current, IOUT is approximately VTest_In(AC) / 2 kΩ."

    Based on this input current generated by Test_In and your transimpedance gain, you'll find your expected differential output, "OUT+" - "OUT-". Because the OPA857 is psuedo-differential, "OUT-" is set to mid-supply, while "OUT+" will vary to comply with the differential output.

    Let me know if you have anymore questions on this. You can find more details of this internal test circuit at section 7.3.3 of the OPA857 datasheet.

    Hasan Babiker
  • I think on TIDA-00725 Signal-Chain board ( , is not possible to connect Test_SD to Vs. As on this PCB I don't have access to cut the tack between GND and Test_SD. If I use jumper then it will short the supply. Is it correct?
  • Hey Ahsan,

    Sorry, I thought you were referring to the OPA857EVM not the TIDA-00725. Please refer to the following thread:

    Hasan Babiker