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Original question:

OPA3S2859-EP: TIA: Gain switching Saturation Issue

OPA3S2859-EP: Getting 600 mV output Voltage and 120 mA current all the time

Niman Munasinghe
Prodigy 10 points

Part Number: OPA3S2859-EP

Hello,

This is my first time posting in this forum. I am attempting to design a high-speed transimpedance amplifier circuit using the schematic provided in the datasheet. However, after assembling all the components, the circuit does not output 0 V when there is no input signal.

I am utilizing a rail-to-rail power supply of 2.5 V / -2.5 V. Even after adjusting the gain settings, there is no change in the output. My understanding of amplifiers is limited, and I would appreciate it if you could review the circuit below and assist in resolving this issue.

  • 0
    TI__Expert 6050 points

    Hi Niman,

    One thing that I immediately notice from the schematic you provided, is that you have left INA+ floating. Can you tie this pin to ground so that the input pin has a voltage reference?

    Also, when you say that you are applying no input signal, do you mean that you have the photodiode connected but you are not exciting it; rather it is only supplying a small DC bias dark current?

    Can you also share what SEL0 and SEL1 configuration you are using?

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 10 points

    Hi Nick , Thanks for the repsonse. I thought its not neccessary to tie those input pin to ground. I will do that . 

    No input signal means , without connecting the Photodiode . If there is no input signal at the comA , should output give 0 V volatge ? but my circuit its giving around 600 mV and 200 mA output with no input. 

    Configuration is SEL 0 : Low   SEL1 : Low

  • 0 in reply to Niman Munasinghe
    TI__Expert 6050 points

    Hi Niman,

    Thank you for providing the additional details. If there is no input signal at the comA pin, it is my understanding that the output would be close to 0V (with a small offset voltage that is inherent of the amplifier... but that offset voltage should be much smaller than the 600mV you are seeing). I will look to confirm that this is in fact the intended behavior.

    In the mean time, if possible, please test with the INA+ pin tied to ground as I believe this may be the root of the issue.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 10 points

    Hi Nick , 

    Thanks for the response . I did tied INA + pin to ground. its still around 500mV . input drawing current is 30 - 40 mA ( Vs+ and Vs-)

  • 0 in reply to Niman Munasinghe
    TI__Expert 6050 points

    Hi Niman,

    With the configuration that you have, a voltage offset of 0.5mV along the 1kohm feedback path would result in an output of 500mV. The device can have a voltage offset of 0.5mV so this output may be expected. Also, the Iq of the device is typically 44mA so that behavior also seems to be as expected.

    What is the behavior of the device with the photodiode connected?

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 10 points

    Hi Nick , 

    I couldn't test with Photodiode . My last photodiode was blown and I am waiting for a new photodiode . I will test with a new photodiode and get back to you. 

    If we change the Configuration to SEL 0 : Low   SEL1 : Low , that means 10k feedback resistor path is using. But its still giving 500 mV output and input currents are same . is this normal behavior ? 

  • 0 in reply to Niman Munasinghe
    TI__Expert 6050 points

    Hi Niman,

    Can you clarify one thing: when you say that the photodiode is disconnected, is the COMA tied to VEE or is it left floating? 

    Also, is this a use case where you plan on having no input signal?

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 10 points

    Hi Nick , COM A is left floating . But I tied it to a ground and checked,  its still the same . 

  • 0 in reply to Niman Munasinghe
    TI__Expert 6050 points

    Hi Niman, 

    Could you share your board/layout files? 

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 10 points

    yeah sure. KiCad files. 

    Niman.zip

  • 0 in reply to Niman Munasinghe
    TI__Expert 6050 points

    Hi Niman,

    I am looking into this behavior and I look to have more answers by the end of the day tomorrow to give you.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    TI__Expert 6050 points

    Hi Niman,

    It is possible that the device has been damaged. In order to check this, is it possible for you to swap a new good device into the board?

    Also, before running with a new device, can you make sure of a few things:

    • INA+ is tied to GND
    • LTCHA is tied to VCC
    • SEL0 and SEL1 are set to '00', '01', or '10' to select an external feedback path ('0' being VEE, '1' being VCC)

    The circuit should look similar as shown below

    Please also connect Channel B in a similar fashion. You can leave COMB tied to VEE or leave it NC.

    Thanks,

    Nick