I have noticed that the transimpedance is too much for my application. Is there a way to reduce the transimpedance externally.
The transimpedance is HMC799 is fixed. To reduce gain two ways might work, depending on your application.
Reducing the gain of the connected photodetector
Adding an external attenuator to the output of the HMC799.
Can you briefly describe your application?
thank you for your reply. The TIA is uses in a laser receiver application (distance measuring system).
I think the problem is that the photo current ist too big in some situations, so that the TIA is going to the limitation and then the phase shift will be too big.
The output voltage is about 3.6 volts_dc (4.7 v_dc supply voltage). I thought about to shift the operating piont of the TIA by voltage adjustment on CEXT. So that the dc voltage point on output is reduced. But i think so all operating points are shifted and the TIA don' t work as desired.
What is the minimum current you are expecting? If you are experiencing too much return, reducing the bias of photodiode or inserting an parallel resistor(~1k to 500ohm & a DC block) would reduce the gain. Are you using pulsed operation or CW operation? what is your bandwidth?
thanks for your response.
The minimum curret i am expecting is less than 100 nA.
Unfortunately i can not reduce the bias, because it will increase the distance noise of the measurement.
I am using CW operation with a BW of about 500 MHz.
The thing with inserting a parallel resistor sounds interesting.
But i think i will lost very small signals when i measure far distances on dark surfaces.
So i will try it and get a positive result.
Thanks a lot
Thanks for the feedback, if you are expecting 100nA signals, using a resistor doesnt sound good. What you are experiencing is a typical optical receiver problem which requires high dynamic range. In these cases, I suggest you to use both linear and limiting regions of the TIA. This allows you to operate on a higher dynamic range. You can use two ideas below:
- If it is possible adaptively increase or decrease the transmitter power so that the TIA will be in the linear mode
- Using a low pass filter with cutoff around 1GHz would transform the signal again to sinusoidal signal(assuming that your applicaiton is similar to FMCW Lidar and you are feeding this signal into a mixer)
We can discuss different ideas if you wish
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