[time-nuts] Multiple Voltage monitoring

Hal Murray hmurray at megapathdsl.net
Fri Jul 31 06:24:17 UTC 2009


> Does anyone have a good solution for monitoring 4-8 different voltages
> at the same time?  I only need a sample rate around 1-2 Hz, but would
> need GPIB, serial, or ethernet support with protocol documentation.

Which end of the $$$ scale are you interested in?  Are you willing to spend 
cash to save time, or do you prefer to invest time (hacking) to save $?

> DC, 0 to 6V, 10mV or better resolution.

10 mV out of 10 V is 10 bits.

Your 6V requirement is a bit ugly.  Be prepared to add an external divider.

You can get 10 or 12 bit A/Ds on single chip micros.  (Or could several years 
ago when I was last paying attention.)  So I expect somebody in that business 
makes something that will solve your problem without breaking the bank.

For example, Atmel's ATMega8a has 6 or 8 10 bit A/D channels.  (6 on DIP, 8 
on other packages.)  The data sheet says 15 k samples per second, but that's 
the marketing number.  I gave up before I found out how slow it is when you 
switch channels.  It's unlikely to have troubles with 1 or 2 Hz.

I expect Microchip/PIC has something similar.  Atmel may have something 
better, I didn't look very hard.  The Mega8 was just a sanity check.  (I'm 
biased, I've used it before.)


A/Ds (generally) fall into 2 categories.  I'll call them fast and slow.  The 
slow ones are linear in the number of counts rather than the number of bits, 
so they can be really slow when you get to 16-20 bits.  Think of the refresh 
rate on a typical DVM.  So you probably need to avoid those.  (unless your 
1-2 Hz is per single reading rather than getting a reading on each of the 4-8 
inputs)

The fast ones are linear in the number of bits.  (Then there are the 
super-fast, flash converers that are just 1 cycle and maybe a few pipeline 
delays.)




-- 
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