I am at that stage of a PCB design where I need some guidance on guard rings.
The application is a photodiode circuitry (using a pin photodiode and transimpedance amplifier.) The amplifier output polarity is (-)ve. This is then converted to a (+)ve signal by a unity gain inverter. The inverted output is connected to other amplifier stages of varying levels of gains.
The levels of current that I am trying to measure goes from approx. 0.5pA to 50uA. The supply is +/-5V. The photodiode is in "zero bias mode" and connected directly across the amplifier. To keep the gain resistor values down, the circuit is operating in a differential mode. See the picture below:
The question is directly related to the transimpedance amplifier circuit. I know that in order to measure these small currents, I need to reduce the leakage currents to a minimum. In addition to that, I need to use lpamps with bias currents lower than the level intended to be measured. I have found a couple of lpamps from TI and AD (don't worry about the OPA192 in the circuit as this is not the amplifier that is used,) but I need to add guard rings as well.
This is the current layout with the guard ring implemented (top layer.)
Guard ring inner layer 1, inner layer 2 & bottom layer:
Is this ok? Do you have any suggestions that could be implemented?
I have re-done the PCB like this:
This has been replicated on the other layers as well. As you can see all the guard rings have been connected to each other using vias and they connect to a single point (noninverting pin of opamp).