LMC662: Opamp Guard Polygon :Only in Top PCB layer or or all layers

Hello, 

Thank you for your question.

Generally, it is most important to identify the primary contributor to any leakage currents when designing a guard ring. In the case of surface mount devices, a single layer guard ring on the signal layer will usually suffice. For through hole components, it is recommended to implement a guard ring on multiple layers. 

This three part EDN covers design and layout of low leakage circuits in great detail: https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-one/

Depending on your circuit design, you may be able to employ a similar guard ring design to what is used in the LMP7721 EVM

Please let me know if you have any questions,

Regards,

Jacob

Hi Hari,

remember why we are using a guard at all: Setting the guard on the same potential as the signal to be guarded reduces the potential difference between the guard and the signal to be guarded to zero and by this the leakage current flowing to and from the signal to be guarded to zero as well, according to:

I = U / R = 0V / R = 0A

So a guard can only work properly when it completely surrounds the signal to be guarded. And because of this, extending the guard in deeper layers of your printed circuit can absolutely make sense.

Kai

Hi Kai, 

I certainly agree with your reasoning for using multiple layers of guarding. There are many cases where multiple layers of guarding are required to effectively reduce leakage currents. 

Hari,

I do not recommend applying solder mask to guard rings. By removing the solder mask over the guard, you further reduce the paths that leakage currents can take. 

Note, surface contamination of the PCB can significantly reduce the effectiveness of guard rings. I recommend thoroughly cleaning the PCB before and after assembly. 

BR,

Jacob