• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 46 subscribers
  • Views 395 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

opa657 transimpedance mode

sama simi
Prodigy 10 points
Other Parts Discussed in Thread: OPA657, OPA847

i am trying to use opa657 in the transimpedance mode. I have used RF of 100 kohm and CF of 0.2 pF. But when I give a light pulse of 50 nss, the output at pin 6 rises slowly while the voltage at pin 2 is not zero. Ideally it should be zero. The voltage at pin 2 goes low and then rises slowly. pls help.

  • 0
    TI__Genius 17905 points

    Hi Sama,

    I have some questions/requests about your circuit:

    What is the photodiode capacitance? A 50ns pulse can be taken as a 100ns period square wave which would have a fundamental frequency of 10MHz. If the photodiode capacitance is around 5pF (and adding the input pin capacitance of around 5pF for a total of around 10pF source capacitance), the achievable transimpedance bandwidth will be on the order of 16-20MHz. This is based on calculations using the equations in the following application note:  Transimpedance Considerations for High-Speed Operational Amplifiers - sboa122. Since you are dealing with pulses, you will want a much wider bandwidth than 10MHz to better capture the transient response of the pulse. The OPA847 may be a better choice of op amp for your application with its higher bandwidth. Alternatively, you might consider reducing the transimpedance gain of this stage and adding a second amplifier stage to amplify the signal further.

    What is the expected current levels of the photodiode?

    Can you provide more detail when you say pin 2 (inverting input pin) goes low then rises slowly? What voltage levels are you seeing? Also, probing transimpedance circuits is a bit tricky - if you probe the inverting pin with an oscilloscope, you are effectively adding the probe capacitance in parallel with the photodiode source capacitance, so the measurement may not be accurate...

    Can you please send a schematic of your circuit with all the component values and supply voltages labeled?