How to sample multiple signals at the same time with ADC?

You could use a multiplexed A-D and front end sample hold ICs - it's possible to make your own, of course, but unless you are trying to squeeze the last cent from the design or some other special reason, I'd go with a premade one.

16 bits of resolution implies 0.001% accuracy for all errors. Even the best ICs I could find (Analog devices) spec 14 bit accuracy (and are expensive - $9 - $34 each depending on which one you choose).

Much cheaper is the LF398 (298/198) [A] range from National

Has 0.002% gain accuracy, [15 bit accuracy without including other errors], so droop will be the major limiting factor.

You could probably get 12 bits [enob] out of this without much trouble.

Cheers

PeteS

Hi PeteS - thanks for the response. I didn't realize there were chips designed just for this purpose. The National part looks like exactly what I want. The droop shouldn't be too serious of a problem as long as I have a fast ADC running. (currently looking at the TI ADS1258). Do you think that my ADC readings will be inaccurate if I have the ADC on a board separate from the sensors? Specifically - I need the sets of sensors at right angles to each other - so there will be three boards all orthogonal to each other - and I'm thinking just one of those boards would have the ADC on it. They would then be connected to each other with right angle pin headers or something like that. The trace length from the ADC to the sensor should be under 4cm.

Thanks!

-Mike

While it is doable, maintaining 12 or more bit accuracy across a connector is increasingly problematic as bits increase. I would put ADCs on each board. Designers choice though.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

Hmm in that case... the Analog SF7656:

doesn't look too bad at all. Expensive part ($17 each for 1K parts) but the added simplicity of the part might make a whole lot of sense. I could put one on each board and be done with it - no muxes, no worrying about signal degradation between parts and boards, no sample and hold chips. Just a shared SPI interface between the three boards.

-Mike

I agree with Joseph, and the choices look nice. My general rule is it's best to keep the signal in the digital domain as much as possible. It's much better to run digital signals across connectors than analog ones, and will make the connector much cheaper than you would otherwise need.

Your droop issue will depend on the timing of the hold signal to the track/hold to the time the last sample is fully converted (or latched if the A-D has an internal hold cap). That will be dominated by the leakage of the hold amplifer, the hold cap (use a nice one, *not* a ceramic - there was a thread on such things recently) and the internal droop, leakage and input resistance of the A-D.

Cheers

PeteS

Hi all,

The problem here is not the sample&hold but the time required for reading the 12 holded values with a reasonable accuracy (as fastest as possible after sampling is finished) using a multiplexed ADC.

regards, Vasile

Mike No> > You could use a multiplexed A-D and front end sample hold ICs - it's

You say 'at the same time' and then 'within 0.5 msec'. You can do a lot of processing in 0.5 msec. For instance the AD7651 will do 16 bit conversions on single channel at 100 KSPS rate, so you could multiplex

12 inputs and still get around 8 KSPS on all channels. Thats far faster than your 0.5 msec cycle time, and a much cheaper part.

--
Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
Design Engineer         J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

Yes, watch the hold capacitors closely, i prefer polystyrene, polypropylene, or mylar(PET). And not "Stacked".

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller