LMH6629: Gain and Phase Measurements of the TIA circuit

Hello Ratul,

  Thank you for reaching out. We have done similar setup in the lab without a photodiode or AC input current source. The method we use is highlighted below:

  This is from a PowerPoint with more details attached below. Make sure that C1 is your estimated photodiode capacitance you are planning to use, and then LMH6629 should be setup in a standard TIA configuration with the appropriate feedback capacitance for stability.

   We have developed a tool for designing TIA circuits with photodiodes. You can modify multiple input parameters like output voltage range, diode current, selection of opamp, etc. At the output you can see transient + frequency response, noise analysis, etc. If you want a starting point for Cf, please take a look at the TIA circuit design tool: Analog circuit design studio, which will recommend a Cf (and also best op-amps) for the required circuit conditions. Also if your circuit is not following the minimum stable requirement, a warning will pop up in the bottom right window.

3326.Transimpedance design flow using high speed op amps.pptx

Thank you,

Sima

Thanks! I don't have a photodiode for my applicaiton. In that case, what should be the C1 and C2 values? I have done TINA simulation of the circuit with LMH6629 with just an AC current source at the non inverting input.

Hello Ratul,

  Sorry for the delay in response. Are you going to be using a photodiode in the future? If not, I would set an example low input capacitance value of 1pF. But, would need to make sure CF is set to a value where (Cinput/Cf) + 1 > 4 since the device is only stable at gains greater or equal to 4V/V. 

  Here is an e2e thread on this same topic with more details. The links in the response do not work, but the summary I gave after the response is the same information: e2e.ti.com/.../opa818-testing-tia-without-photodiode

  Please feel free to ask any questions for more clarifications from above link.

   Also, as a simpler quick method, you can use the same values as below from LMH32401EVM, which shows how to test with a voltage source if no current source is available. The LMH32401 as integrated feedback capacitance to keep amplifier stable, this will need to be added for LMH6629.

Thank you,

Sima