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OPA857: Could not get an output

Part Number: OPA857
Other Parts Discussed in Thread: OPA693, OPA858, OPA855

Tool/software:

I am designing an amplifier to amplify the signal from a photomultiplier tube (PMT) and digitize the output using a standard digitizer.

I used a circuit incorporating the OPA857 and OPA693, but I am not getting any output signal. While adjusting the offset with the potentiometer, I can observe changes in the DC voltage level, but no signal is present.

I would greatly appreciate any assistance in identifying potential issues with the circuit or improving its design.

  • Hello Bhaskar,

      Thank you for the information and debugging steps. I double-checked your connections, and they are correct. I have a few more quick questions:

    1. What is the PMT's capacitance
    2. Is the bias of the photodiode with the cathode connected to a positive supply, and the anode connected to IN pin of the OPA857? These connections make sure that the photodiode sources an output current that results in the OPA857 output swinging down below the reference voltage = (5 / 9) × 3.3 V = 1.83 V.
    3. Are you getting the right supply current?
    4. Can you probe at OPA857 output to isolate the issue or confirm if issue is from first stage (which I assume due to potentiometer working on second stage), recommended if you can place a isolation resistor between the two amplifiers. Also, can you probe OUTN to see if you get around 1.8V to confirm device functionality? 

    Best Regards,

    Sima

  • Thank you for the suggestions.

    1. What is the PMT's capacitance?
      Anode capacitance is approximately 3.3 pF.

    2. Is the bias of the photodiode with the cathode connected to a positive supply, and the anode connected to IN pin of the OPA857? These connections make sure that the photodiode sources an output current that results in the OPA857 output swinging down below the reference voltage = (5 / 9) × 3.3 V = 1.83 V.
      We are working with a PMT.
      The cathode of the PMT is connected to the high-voltage negative supply, and the anode of the PMT is connected to the INPUT of the OPA857.
      The PMT sinks current at the anode (is this the reason why it does not work?).
    1. Are you getting the right supply current?
      Using an oscilloscope we can see the signal. Besides, using commercially available amplifier (TIA60, Thorlabs), we can acquire the signal/image.

    2. Can you probe at OPA857 output to isolate the issue or confirm if issue is from first stage, recommended if you can place an isolation resistor between the two amplifiers. Also, can you probe OUTN to see if you get around 1.8V to confirm device functionality? 
      OUTN gives 1.81V.
      OUT gives 0.268V persistently.
      We removed the second stage OPA693, result seems the same.
      Perhaps, the issue might be in this first stage itself?
  • Hello Bhaskar,

      Thank you for the update on each suggestion. I agree that 2. is the reason why you are not getting an output from the device with input current change. This device was designed to work from the internal reference voltage (1.83) and below. The output voltage range of the device is from 0.6V to 1.9V on a 3.3V single supply configuration. This 1.2-V swing corresponds to a maximum input current of 60 µA in the high-gain configuration, and 240 µA in the low-gain configuration when diode is sourcing current (cathode is biased to a positive voltage and anode is tied to the amplifier input). It forces the output to go low, below the device's internal reference (in other words, negative output when light falls on the PD). 

      Are you able to switch out PMT that works with positive high voltage supply? If not, are you able to switch out the amplifier with a non-integrated TIA since you are not using the OUTN functionality, such as OPA858 or OPA855? 

    Thank you,

    Sima