- Joined Jul 15, 2018
This circuit is a simple unity gain signal decoupler using an optical isolation chip. The left half of the circuit is a feedback system to account for fluctuations in the LED. The right half then translates the second photodiode's voltage back into the input voltage.
My misunderstanding is, for either the left or right side, how the op-amps can bring V- to ground (since V+ is grounded) by feedback when the photodiodes are directly connected to V- without resistance in-between. I don't know the way to approach this analysis.
To narrow my question, lets say that some input signal was producing oscillations on the second output diode (on the right). The op-amp does not rail. Does the op-amp just produce an equal an opposite voltage at that position in order to cancel by superposition? This intuitively seems wrong.
Some important things to note:
-The LED and photodiodes have a small capacitance on the order of 10's of pF.
-The circuit assumes the input is a positive only AC signal.
-I have tested the circuit. It works at unity gain as expected until some upper roll-off frequency.
The reason I still analyse the circuit is because it filters out higher frequencies, but I am finding trying to understand the theoretical frequency response so I can compare it to what I have measured.
Thanks in advance!!