Guard rings

[Carl]

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I have very limited experience in PCB layout. I understand that guard rings are often used around the input pins on JFET (& I guess now CMOS) input opamps. But I don't understand how they affect leakage current.

It would seem to me that guard rings would only provide some level of AC isolation, and then mostly if you ground the guard ring. Also, it would seem that you would want to guard ring the whole trace(s) that connects to the pin, and not just the pin itself. And, with the low frequencies dealt with in detector-land, I would even question the usefulness of all this.

Can someone give a detailed explanation of what guard rings do? Do you use a common guard ring around both the + & - inputs? Do you ground the guard ring?

- Carl

[Dave J.]

Re: Guard rings

The purpose of a guard ring (also called "guard band" or simply "guard") is to intercept surface leakage current and to shunt it to ground or other harmless node such as the op amp output.

I have heard of embedding guard rings internally, to intercept bulk leakage currents, in PC boards by having an array of vias surrounding the sensitive node like a picket fence.

--Dave J.

[Dave J.]

electrostatic fields; solder mask

Guard rings etc. also help shield from unwanted electrostatic fields, both AC and DC, but they are not nearly so effective for that purpose as they are for intercepting surface leakage.

Strictly speaking, a guard ring on an etched PC board should not be covered with solder mask. However, this detail is often overlooked, and satisfactory results are still often obtained. I usually attribute the satisfactory results to a clean board and low relative humidity (no leakage to intercept, guard ring ineffective but it doesn't matter); however, if the bulk resistivity of the solder mask were no higher than that of the PC board material, and the thickness of the solder mask were much less than the width of the guard band, then the guard band might still intercept enough current to be better than useless.

--Dave J.

[pawel]

Re: Guard rings

I have seen a nice explanation and some examples in the National LMC662 opamp datasheet, but of course a lot of othes sources exit.
Regards,
pp

[Sean Goddard]

More on guard rings and PCB design.

Another use of guard rings in the beeper application, is to minimise the phase shifts and "phase noise" introduced by changes in humidity by providing a predictable capacitance and leakage path at the amplifier input.

Correctly done, the input resistors should be mounted on PTFE stand-offs, and one end (the end connected to the amplifier pins) should also be enclosed within the ring.

BOTH positive and negative inputs of the op-amp should be enclosed within the ring, which should be on both the TOP & BOTTOM layers of the PCB. Through board vias, as Dave J. has said, arranged like a picket fence, will also help, as will coating the whole thing in a military grade conformal coating (don't forget to bake the PCB for 24 hours at about 60 degrees C to remove any moisture prior to spraying).