Subject: Re: ground loops, optoisolators, and ADC noise, oh my!
Date: Wed, 22 Jan 2003 01:45:02 +0000 (UTC)
NNTP-Posting-Date: Wed, 22 Jan 2003 01:45:02 +0000 (UTC)
User-Agent: tin/1.5.14-20020926 ("Soil") (UNIX) (Linux/2.4.20 (i686))
Steve T Shannon wrote:
> I've been working on my first mixed-signal board for about 9 months
> now, and just when I was really excited that things were going to work
> (the amplifier section exceeds design specs!) I soldered on the first
> of my ADCs and found... that the conversion process generates
> fantastic noise. The ADC's input, when running, shows literlly 10-20
> mV of noise on each conversion cycle. This is with a scope probe --
> the digital output shows the same noise, of course. Now, this board
> has multiple simultaneously-sampling ADCs, so I run the /CONVST line
> to all of them from the output of an FPGA (Sampling below 300 khz). My
This almost certainly isn't where your noise is coming from.
There are several likely possibilities IMO.
Ground bounce. All the ADCs starting sampling at once may use a large spike
of current, this can disturb your ground reference.
Check decoupling and ground seperation.
Charge injection. ADCs are not perfect. A common imperfection is that
some charge gets injected back into the input pin, this
causes the voltage on the input pin to vary depending on
the impedance. Check your buffering with the manufacturers
recommendations. Faster op-amps, or a lower output impedance
Coupling Some either electromagneic or electrostatic coupling between
possibly the supply rails due to large circulating currents
when the ADCs are triggered, and the input amplifiers.
This is worst if you are using things like thermocouples.
I'd bet on number 2.
http://inquisitor.i.am/ | mailto:firstname.lastname@example.org | Ian Stirling.
Things a surgeon should never say:
Better save that for the autopsy.