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OPA857EVM-978: Why am I getting 0V at both outputs (VOUTN = VOUTP = 0V)?

Samuel Phiri
Prodigy 30 points
Part Number: OPA857EVM-978
Other Parts Discussed in Thread: OPA857

Hello,

I want to use the OPA857EVM-978 in the photodiode mode with a SM05PD1A Silicon photodiode. 

Test setup
- Evaluate the performance of the OPA857 in the standard photodiode input mode

○ The test mode is disabled: Test_SD = GND, Test_In = +Vs 

§ Connected SH-J4 to J4 to set Test_In to +VS
§ Connected SH-J5 to Test_SD (middle pin) and GND

- Measured node voltages

○ Photodiode output voltage: 0.59V
○ Vcc = +3.3V
○ Voltage before C7 (3.9pF): OUT = 3.097V, OUTN = 1.88V, (these voltages are both 1.88V when the photodiode is not connected)
○ Voltage across R8: 2.489V
○ Input voltages to the transformer: V1 = 2.489V, V3 = 2.489V
Transformer output voltages: V6 = V4 = 0V --> VOUTN = VOUTP= 0V

1. Why is the output (OUT) from the OPA857 3.09V? I thought this is optimized to swing below the internal common-mode reference voltage (1.83V).

2. Why is the transformer output 0V? 

Best regards,

Sam

  • 0
    TI__Mastermind 22270 points

    Hello Sam,

        1) You are correct that the signal should be between 1.88V and 0.6V. That is very strange, the output swing should not go above 1.9V. One concern I do have regarding you circuit configuration is the photodiode. It looks to be grounded at the cathode, you should provide a few volts of reverse bias n order to get an output below 1.8V. However,  for this issue, it would only show an output steady at 1.8V and not dipping below this level. Would you be able to follow the procedure listed in section 5 in he user guide using the Test Mode instead, and let us know your findings at the same voltage outputs you provided? 

        2) Since, you are getting the same input voltage in magnitude and polarity at the output, you will get a 0V output once combined at the 1:1 transformer. 

    Thank you,

    Sima

  • 0
    Guru 140200 points

    Hi Samuel,

    a transformer cannot transmit DC voltages.

    Kai