How to treat COM pin of ADC? - Page 1
Forum section: electronics / beginners

author: dusan
author_rank: Regular Contributor
timestamp: June 15, 2025, 01:14:05 pm
content: What is the correct way to wire COM pin of ADC? I've worked with few arduinos that don't have separate analog ground and often ADC wasn't very accurate especially towards zero as it was picking various other currents from shared digital ground. So I got ADC with separate COM pin (ADS7828E) and now I want to wire it in a way that minimizes digital noise. Here is my schematic, COM pin is not connected yet.And now my question is how to use this COM pin? I can think of at least few different ways how to connect it:1. ADC COM pin to the nearest ground (e.g. A0 pin) and J10 COM pin underneath the ADC to COM pin2. ADC COM pin to the nearest ground (e.g. A0 pin) and J10 COM pin to pin 9 of ADC3. ADC COM pin to the ADC ground pin (pin 9) and J10 COM pin also to pin 9 of ADC4. ADC COM pin to the J11 pin 4 and J10 COM pin to J11 pin 45. ADC COM pin to J10 COM pin, neither connected to GND6. Something else

author: shapirus
author_rank: Super Contributor
timestamp: June 15, 2025, 02:34:00 pm
content: Separating (and how exactly), or not separating AGND and DGNG is a complicated topic and full of myths, too.I once found a nice talk with Rick Hartley on this, here you go:

author: xvr
author_rank: Frequent Contributor
timestamp: June 15, 2025, 05:32:53 pm
content: COM pin of ADS7828E is not AGND. It is negative input of ADC (yes, this ADC has is some kind of differential input).It should be connected to GND point of signal source (and difference from this signal ground and ADC GND should be in range +/- 0.2V)

author: jwet
author_rank: Frequent Contributor
timestamp: June 15, 2025, 06:42:06 pm
content: As xvr said, COM is not a ground. What it is really is another input that can be used with any input to sample the voltage between its potential and any other input. It is always a negative input relative to the other input. These parts sample the input with a mux and provide the ADC with a charged cap with the voltage to be converted. That's what the "sampling" in the name means.If you look at table II, you'll see that "any" input can be used like COM. The input is a big Mux that can sample any input against com or any other input against and the polarity can even be swapped!; which has a few nice features- like nulling offsets.(see below).Table II left top has a 4 bit input shown, the first bit SD is a single ended bit and when high samples each input against com which can be floating at some pseudo ground, etc. If SD is low, you can sample the difference between any two channels either positive or negative. This can be nice for eliminating offsets- take two readings of opposite polarity add them with sign and you'll get your offset that you can back out.These features were put into modern mixed signal stuff to support single supply operation and increase flexibility. Back in the day, analog generally ran on +-15V supplies and standard signal ranges were +-10v. ADC's being mixed signal (analog and digital) needed to have dense digital and HV analog- not an easy combo, a lot of old ADC's used +-12v analog rails plus a digital 5v (eg AD676, etc) to minimize this HV need. As progress came, Vcc's shrunk and now high precision single supply analog is well established. You'll occasionally see +-5v in high performance systems.

author: xvr
author_rank: Frequent Contributor
timestamp: June 16, 2025, 12:31:39 pm
content: QuoteYes, but with caution. 'Electrical characteristics' table says that negative input of ADC has absolute input range only from -0.2V to +0.2V. Positive has much larger range: from -0.2V to Vdd+0.2V

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 16, 2025, 02:06:37 pm
content: Quote from: xvr on June 16, 2025, 12:31:39 pmWell-spotted. This is really a strange part (in a bad way). It advertises operation in both single-ended and differential mode, but even in differential mode the negative input has to be within 0.2V of GND. That's not at all what I would call differential mode.Many other ADCs with similar pinouts do support arbitrary voltages on both inputs to the ADC, and can be configured to provide unsigned (for single ended) or signed (for differential) output codes. I'd suggest looking for a newer ADC, you should be able to find some with more flexibility, better performance, and probably a lower price than this.

author: tszaboo
author_rank: Super Contributor
timestamp: June 16, 2025, 02:35:35 pm
content: The reason to have these it to be able to lift the negative input from ground. If you have operational amplifiers that work on the 0V-5V rail, even if they are rail to rail, still they are not going to be able to output exactly 0V. So you lift the AFE's ground to say 100mV. Any signal on the input goes into a difference amplifier or Instrumentation amplifier, that work on +/- rails, gets divided if necessary, and your ADC drivers are operated on the same 5V rail as your ADC.Quote from: mtwieg on June 16, 2025, 02:06:37 pmIt is a 25 year old part. Which doesn't mean it's bad, it just doesn't have all the bells and whistles as the parts made today.

author: wraper
author_rank: Unknown
timestamp: June 16, 2025, 02:50:14 pm
content: Quote from: shapirus on June 15, 2025, 02:34:00 pmThe simple answer is that if you don't really know what you're doing, then effing don't separate GND. I've seen annoying hard to catch random issues, IC latch-up once-twice a year in $40k-200k laboratory devices from different manufacturers that were caused by thoughtlessly separating ADC GND. In all those cases the fix was running a wire between AGND and GND, and issues were miraculously gone for good with no performance degradation.