[SI-LIST] Re: ADC Gnd Noise
- From: Dave Instone <Dave.instone@xxxxxxxxxx>
- To: kd_kamath@xxxxxxxxx
- Date: Thu, 12 Apr 2007 15:04:47 +0100
When you say AGND w.r.t AGND ect. where exactly are you measuring it? at
the ADC pins? somewhere close to that? how long are the leads that
connect scope live and ground? All these will affect what you see on
the scope. If you have one use a differential probe, if you haven't
tried one before for this type of measurement it will really open your
eyes to how far out a single ended probing method can be. Has your
scope got an FFT capability or can you feed your probe output into a
spectrum analyser, this can really help in deciding where that 'noise'
is coming from.
Regards
Dave Instone
+44 (0)1235 824963
OXFORD SEMICONDUCTOR LIMITED
25 MILTON PARK
ABINGDON
OXFORDSHIRE
OX14 4SH
Registered in England no 2733820
Registered Address: As above
deepak kamath wrote:
> hi,
> Thanks for the suggestion. We have measured analog ground noise with
> respect to digital ground and it is around 300mV(p-p) on the scope. Even if
> we subtract 60mV noise due to probe, the noise will be around 180mV(p-p).
> When there is no signal to ADC, the 8 LSB bits from ADC are toggleing. That
> is, ADC is seeing around 80mV(p-p) noise at its input.
>
> The clock of 7.414MHz is generated using PLL inside the FPGA and is passed
> through a 3.3V to 5V transceiver having driving capacity of 25mA. The clock
> output from the transceiver is routed to ADC without any sheilding. We are
> observing overshoots and undershoots of about 2V(p-p) on the ADC clock, but
> there is no shift in the edges of the clock.
> Yes, we have checked both grounds and we are observing the noise of around
> 300mV(p-p). That is, AGND w.r.t AGND has 300mV(p-p), DGND w.r.t DGND has
> 300mV(p-p) and AGND w.r.t DGND has 300mV(p-p).
>
> Thank you and regards,
> Deepak.
>
>
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