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OPA657 problem_power supply

Guangjun Gao
Prodigy 65 points

Other Parts Discussed in Thread: OPA657, TINA-TI

Dear all,

I am using OPA657 as a transimpedance, but when I apply the +-5V to pin 7 and pin4, and ground to pin3, I found the voltage on the pin4 changed from -5 V to about +0.8V, and the output of amplifier is also not right. 

is there something wrong in my OPA657?

Thank you

  • 0
    TI__Genius 13335 points

    Gao,

    From the description it is hard to tell exactly what the problem is. There are a number of things that could be going on here. If the feedback capacitor is off or the gain is too large, you could be saturating against the rail. When we typically design a TIA, we use something similar to the attached TINA-TI simulation file.

    0537.OPA657 TIA.TSC

    You can find the design equations in the application section of the OPA657 datasheet starting on page 10.

    Also, another question would be how much can your power supply source and what are the current/voltage limits set to? There could be an issue with the drive capability of the supply.

    If you upload your schematic we can take a closer look the problem you are having.

    Regards,
    Luke

  • 0 in reply to Luke LaPointe
    Prodigy 65 points

    Dear Luke,

    Thank you for your kind help. I use the NI-9623 AO output as power supply. The parameters of power supply is : +-10V (maximum), 10mA, and the connection chat is as follow,

    I simulate with TINA-TI, there is output signal, but for my experiment, the output is about +2v (when +-Vs pin a connected with +5v and -5V) and +0.9v (when +vs connect to +5v, and -Vs connect to ground). Because my major is not electronics, I feel confused about this.

    Thank you. 

  • 0 in reply to Guangjun Gao
    TI__Genius 13335 points

    Gao,

     There are a few things I notice here. The first is that your power supply can only source 10mA. The typical quiescent current of the OPA657 is 14mA so you would be driving the power supply into current limit, which would cause your supply voltages to dip. I looked up the part and the NI-9623 is actually only a +/-1mA current limit per channel! There is also the 10mA version (NI-9269), but in order to properly test this, you’re going to need a supply with a higher current limit. The OPA657 is a high current output drive (up to 70mA max), so depending on the loading, those are the kind of numbers you should be looking for. Is there a bench supply with a larger current limit that you can hook this up to for properly testing the circuit?

     The second thing I noticed is that your positive and negative supplies do not have their common’s tied together. This will cause the voltage levels to be unreferenced to each other and not give you a real +/-5V. Just connect the common’s together to fix that problem.

     The last thing I noticed is that with a 1MΩ feedback resistor, 20pF is very large. Depending on what the capacitance of the sensor is, you’re going to be less than 0.5pF for a proper compensation cap, probably into the 0.03pF range.

    Regards,
    Luke

  • 0 in reply to Luke LaPointe
    Prodigy 65 points

    Thank you for your kind help, Luke.

    I will order a new power supply with larger current limit, and modify the value of cap.

    Guangjun