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Design >> Analog Design >> PCB layout for an instrumentation amplifier with programmable gain
https://designers-guide.org/forum/YaBB.pl?num=1384720179

Message started by mbmsv on Nov 17^th, 2013, 12:29pm

Title: PCB layout for an instrumentation amplifier with programmable gain
Post by mbmsv on Nov 17^th, 2013, 12:29pm

I have an instrumentation amp circuit similar to that shown in this appnote but with the first stage of the instrumentation amp made out of discrete opamps. The layout has to be done in a single layer (except for the power) with the GND plane. I am concerned with routing two Rg traces to the switch and the resistors. With the best placement I could come up with, they end up forming a relatively big loop around the amp's input pins and a few input components attached to them. I wonder whether this is a problem? Thanks.

Title: Re: PCB layout for an instrumentation amplifier with programmable gain
Post by aaron_do on Nov 17^th, 2013, 4:47pm

Hi,

I don't have much exp with your particular problem, but big loops add inductance and also cause radiation. This could cause coupling to nearby lines. You might try simulating the effects using an EM simulator.

Aaron

Title: Re: PCB layout for an instrumentation amplifier with programmable gain
Post by harpoon on Nov 18^th, 2013, 2:48am

Hi,

RG is a critical net, and should not have any current flow across it.

Any coupling to it and you will mess up your CMRR.

So long traces and large loops, are definitely a bad idea. However, you have other constraints to work with ... so another engineering trade off.

Having said that, discrete opamps are not the way forward these days ... better performance (noise) from a single Instr Amp IC, and would make your PCB layout much easier.

in any case ... good luck !

Title: Re: PCB layout for an instrumentation amplifier with programmable gain
Post by mbmsv on Nov 18^th, 2013, 6:52am

harpoon wrote on Nov 18^th, 2013, 2:48am:

Hi,

RG is a critical net, and should not have any current flow across it.

Any coupling to it and you will mess up your CMRR.

Thank you. That's exactly what I wanted to know!

Quote:

discrete opamps are not the way forward these days ... better performance (noise) from a single Instr Amp IC, and would make your PCB layout much easier.

I won't go into the reasons why discrete opamps were chosen but from the point of view of laying out connections to the programmable Rg it wouldn't make a big difference...