# Designing a guard ring for a transimpedance amplifier

I am at that stage of a PCB design where I need some guidance on guard rings.

The Application:

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:

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).

Any suggestions?

cheers

• $10^{8}$ dynamic range is where I worked and your range is almost exactly where I did. How do you plan to handle that range?
– jonk
Jul 18 at 17:17
• The output of the Transimpedance is connected to 5 different Amplifier stages (all connected in parallel). That is how the whole range is handled. Jul 19 at 8:15
• I started with COTO relays. But later found other approaches using integrators with internal capacitors and switches, such as the Burr Brown DDC112. Might consider the idea. (Though the DDC112 has some internal limitations for max current near 7 uA or so due to a particular aluminization path that wasn't made wide enough for larger currents.) The ACF2101 is another consideration to look at.
– jonk
Jul 22 at 7:26
• Hi jonk, Thanks for the suggestion. I have to admit that I hadn't thought of using a charge integrator for Current to Voltage conversion. This is something that I could test at a different time. For the time being, I have already started working on this design and its at that stage where it is almost complete. So changing the circuitry is not possible. Any suggestions on the placement of the Guard rings? Your idea of the charge integrator is really good. How is it with measurement speed? Thanks Jul 22 at 8:53
• At higher currents your measurement rate will be necessarily fast. At very low currents it will be low.
– jonk
Jul 22 at 8:58