OPA837: TIA needs protection

Hi daniel,

One issue that I saw with your circuit is that it is unstable. Are you seeing random noise at the output or defined oscillations? 

I did the open loop stability analysis and found that the phase margin is ~10 degrees, which is very less (should be at least 45 for stability). Please find the screenshots for stability analysis waveforms and circuit-

To stabilize the circuit, I increased the Cf to 300pF and saw a phase margin of ~45 degrees, which suggests that the circuit is stable. Please see the bode plot screenshot below-

If you want to read about the open loop stability analysis in TINA-TI these are few resources: 

• https://www.ti.com/video/4080235259001
• https://www.ti.com/video/series/precision-labs/ti-precision-labs-op-amps.html (Check stability section)

The TIA bandwidth of the circuit is ~6.8MHz as the photodiode cap is very high (4nF).

Please take a look at the TIA circuit design tool that we have developed, where you can insert all the required input parameters and it computes the output parameters and plots like AC response, transient response, noise analysis, etc. The tool also suggests devices. Also note that this tool is good to narrow down on the appropriate TIA to be used and for more accurate values, one has to simulate in TINA. Tool link: Analog circuit design studio.

Please find the attached TINA-TI stability analysis schematic below-

stabiity tia opa837.TSC

Regards,
Aditya

Thank you very much, Aditya. Do we still need to use the Riso and the BAT54S?

Hi Daniel,

I was not able to understand the purpose of Riso in your circuit, can you please help me understand it?

To understand the use case of the diodes, can you please provide me some more information on these points-

1. What do you exactly mean by IC is getting damaged?
2. What waveform do you get at output?

Regards,
Aditya

I first saw the Riso (isolation resistor) in this post:

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1267573/opa858-transimpedance-amplifier-output-oscillation/4814102#4814102

But I am not sure it is a good idea to use it.

1. Once we turn on the circuit, in less than half hour the amp op is damaged and sometimes the LDO itself. The situation was solved after we put the diodes in the negative input of the OPA837, at the expense of a noisier signal at the output.

2. The waveform is a bi exponencial pulse with 50 ns risetime, 500 ns duration and variable peak value.

Can you recommend a strategy to protect the circuit with compromising it SNR?

Regards,

Daniel

Hi Daniel,

The Riso resistor dampens the potential resonance caused by the trace inductance and the amplifiers internal capacitance. You find the relevant documentation in OPA855 datasheet figure 9-14 page 30. So I believe you can use it to improve the TIA response.

Let me get back to you regarding IC protection with diodes. If possible can you please send the output waveform.

Regards,
Aditya

Dear Aditya,

This is the waveform.

Hi Daniel,

Thank you for providing the output waveform.

• As per your initial comments, I believe the yellow channel is the TIA output, but can you please help me understand what the other 3 channels are probing?
• Also can you zoom in the output waveform to check if there are any oscillations present?
• I think it would help us a lot if you provide the PCB schematic, and mention all the nodes that you are probing, and also what input you are providing.

Regards,
Aditya