[SI-LIST] Re: ADC Gnd Noise
- From: "Jon Keeble" <jkeeble@xxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 13 Apr 2007 07:06:31 +1000
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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