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OPA659: Driven Guard

Part Number: OPA659
Other Parts Discussed in Thread: OPA656


I have an unity gain buffer with OPA656(G=+1V/V) or OPA659 (G=+2V/V).

I want to reduce the leakage current to keep the input to be high impedance. so I should place a copper ring around the non-inverting trace.

since I have a unity gain buffer at the front end of my setup How should I place a driven guard?

I found some strategies to place driven guard like below: (for unity gain buffer I couldn't find anything!) I want to build it so help me practically :)

I'm a little thinking of the second pic!






  • Hi,

    You could use the driven guard ring as mentioned in Figure 2 for your application, although I think it might be suitable for non-inverting gain of 2. For unity-gain buffer application, you might be able to use the below guard ring circuit where the output surrounds the non-inverting input.

    However, I would be extremely cautious about this guard ring approach as you might end up with positive feedback at higher frequencies; especially since the non-inverting input and output traces are so close to each other. I think you might be able to get by with not having a guard-ring as both the OPA656 and OPA659 are FET inputs with greater than 1Gohms of input impedance. So, the input leakage current should be extremely minimal.

    Best Regards,