FreeLists / si-list / [SI-LIST] Re: ADC Gnd Noise

["Jon Keeble" <jkeeble@xxxxxxxxxxxx>]

Oh, and I forgot to suggest you look at the clocks into the converter.
Over and undershoot is a no-no, as is jitter, bad levels, and so on.

Jon

----- Original Message ----- 
From: "Jon Keeble" <jkeeble@xxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Friday, April 13, 2007 7:06 AM
Subject: Re: [SI-LIST] Re: ADC Gnd Noise


> It might be useful to partition the problem (divide and conquer).
>
> Disconnect the signal at the converter and connect the A2D input to the
> local reference ("GND"?)
> If the converter generates nice clean bits then the power supplies to the
> converter are ok.
> If not, either the supply or the reference are the problem.
> - filter the power supply pins with beads, small resistors, capacitors etc
> - make sure the reference is clean (right bypass etc).
>
> If the supplies and reference are clean, then progressively reconnect the
> signal chain.
> You don't need a signal to do this unless the noise is somehow a result of
> the signal being present.
> This is possible, for instance if you have an opamp that goes unstable in
> the presence of signal.
>
> Good hunting!
>
> Jon Keeble
> JK Consulting
>
>
>
> > Hey, interesting problem. An ADC can really shine a light on noise
> > problems. In your case you could almost say the level of noise you're
> > having to deal with is what is showing up in the toggling of your 8
> > lower bits....
> > so 256 x 0.3mv=3D75mv, and not the 300mv you measure with your scope.=20
> >
> > If you haven't already, you might try injecting at the sensor input a
> > clean, pure, signal at several known voltage levels (known more
> > precisely than your lsb bit weight of 0.3mv). Compare the input voltages
> > to the digitized values and see if this 75mv noise stays the same at
> > different levels of input. This doesn't fix the problem, just gets you
> > more data that might provide leads.
> >
> > Best regards,
> > Jim Peterson
> > Honeywell
> >
>

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