preventing SPI noise on ADC

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How can I best prevent noise on the SPI lines communicating
between my ADC (Burr-Brown 16-bit 8-ch ADS8344) and a 5V
microcontroller (AVR ATmega32)?

I've tried to keep the digital lines as far away from the
analog lines as possible, and have a 2-layer design with
seperate analog / digital ground planes.

Should I buffer all the SPI lines?  if so, which buffer
chips would be suitable?
Do I use series resistors?

I've seen some opto-isolated designs, but we'll be running
off of the same power supply for the AD and the
microcontroller and they will only be a couple of
centimetres apart.


Thanks.


Re: preventing SPI noise on ADC
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AFAIK I had similar problems and i fixed it with
TL431 Voltage ref on ADS Vref pin.
Assume that power lines are noiseless, Vcc logic and Vcc analog should
be physicaly separate with two PCB tracks connected to CP power (100uF
Tantalium)

Vlogic  -----------------------------------|
Vanal   ---/\/\/\-------|                -----
         |    10       -----              ----- C2 Power
        ---            ----- C1 Power       |
        --- 100nF        |                  |
         |               | AGND             |
       --------------------               ----- DGND

With LARGE tracks on AGND, DGND and Vlogic. a simple 1order RC filter on
Vanal may improve noise immunity of analog path.

Mux COM input as close as possible to AGND

Finally i placed a high stability 2.5V ref on input and the average of
100,000 samples shows a very good distribution.

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Good Luck, Habib.




Re: preventing SPI noise on ADC
snipped-for-privacy@sympatico.ca says...
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Probably not necessary.
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Probably not necessary.
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If you look at the data sheet,  most SPI ADC converters will show
a sampling method that has the SPI lines quiescent during the
actual conversion and then active again after the conversion to
transfer the data.   There is quite often a faster method which
clocks out the data as the conversion takes place and may actually
use the SPI clock as the conversion clock.  This may result in
more conversion noise.

The best way to get good noise results is to keep the SPI lines
stable during the conversion as much as possible, and use an
internal clock as the conversion clock.  This may reduce your
maximum conversion rate, but life is full of such tradeoffs.

The ADS8344 data sheet discusses these tradeoffs on Page 14.

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Mark Borgerson



Re: preventing SPI noise on ADC
Hi Glenn,

You got some good advice here. Just one more comment: Splitting digital
and analog ground may often not be such a good idea. In theory a split
might work but at several MHz into the noise spectrum which your SPI and
other clock transitions will certainly create, such a separation
vanishes quickly. Then it becomes an uncontrolled matter where these
grounds really meet and by how much.

I never use split grounds unless there is an isolation requirement for
safety reasons, such as in medical gear.

Regards, Joerg

http://www.analogconsultants.com

Re: preventing SPI noise on ADC
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To start with, when the SPI is running to read out the values, then ADC
already has a conversion finished. So the SPI signals to the ADC to
rerad it out are of no convern. However, the signals on the SPI lines
that are not caused by your intent to read out the ADC are of concern.

I went as fas as trying the magnetic isolators from Analog devices,
ADum1400 and such, but they are somewhat overkill.

Yes, series resistors on the SPI lines dampen the reflections.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Re: preventing SPI noise on ADC
And no SPI lines under the ADC device will also help.

Gerard




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