I am now designing a transimpedance using ADA4062-2 to detect signal from a photodiode. The first image is the equivalent circuit.
The signal from PD contains 1kHz sine wave and 2kHz sine wave and their equation are:
(while a is a fixed value set manually and b means the phase of 2kHz signal)
My target is to measure the amplitude and the phase of 2kHz sine wave. As the signal is quite small, I decide to use T feedback network. However, I found that the phase of 2kHz would change a lot, especially when a is set to be near 90 or 270 degree.
I doubted that it may be due to the T feedback network. So I substitute the T network with equivalent resistance(the second image). And luckily the phase change is gone.
But I don`t know why would the T feedback network affect the phase. Distortion? or any other reason? Does any one have idea about it? Thank you very much!