Need info on 'Guard Pin' of ICs

[pgokhale]

Hi!
I came across an IC from Analog Devices - AD549, and happened to see a pin labelled as 'Guard pin'. It is being mentioned that the pin is connected to case.

I would like to understand more on the significance of this pin - its need and how to deal with it.
Also, is guard pin's need same as that of 'guard ring'?? Please advice.

Thanks in advance,
PGokhale.

 

[mfb]

As far as I know, those are used to shield the circuit (or pin, or connection, or whatever) from external fields.

 

[pgokhale]

Thank you mfb :) but if the op-amp is employed in low power scenario - employing only dc currents and voltages, the fields could be negligible. In this case how guard pin makes its usage prominent?

 

[NascentOxygen]

I can picture where an earthed guard pin would be useful as a separation between input pins and pins carrying the amplified output in a sensitive high-gain amplifier. You don't want stray feedback from output back to input.

 

[milesyoung]

The PCB trace that carries the input signal to your opamp isn't perfectly insulated, i.e. there's distributed resistance between it and adjacent traces, between it and the opamp package and so on. Nearby voltage gradients can thus cause current to flow in your signal line which might swamp out your input signal.

You can divert this leakage current away by surrounding your signal line with a low impedance trace that's biased at the same voltage as your signal line. The guard pin allows you to make the opamp case a part of this guard conductor.

The AD549 datasheet includes information about this under the heading "Circuit Board Notes".

 

[pgokhale]

@ NascentOxygen, milesyoung : thank you :) sounds good :)