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

Question

Hi - I'm working on a project where I'm going to have 12 different channels of data that I will be sampling at about 2KHz. I need I think about 16 bits of accuracy, maybe a little less. I would really like to take all these samples at *exactly* the same time. So - I could have 12 ADCs on the board... But that'd be just a tad spendy, to say the least. What I don't care too much about is how long it takes to digitize each signal, as long as all are digitized in under half a milisecond (1/2K seconds). So ideally I'd like a chip that could sample and hold all the signals at the same time, and then go back and individually digitize them. Are there any ADCs that can do this for me? If not - are there any good solutions to this problem? I should mention that I might end up needing only 4 channels, not 12. (I'll have 3 boards with 4 channels each - but I'm not sure if I want an ADC on each board or just one ADC for all 3).

Other details if they matter - the devices being samples will be an Analog Devices accelerometer and an Analog Devices gyro as well as probably a Honeywell magnetometer. Fourth channel is the temperature sensor on the gyro.

Thanks!

-Mike

PeteS · Accepted Answer

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).

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

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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

Mike Noone · Answer

Hi PeteS - thanks for the response. I didn't realize there were chipsdesigned just for this purpose. The National part looks like exactlywhat I want. The droop shouldn't be too serious of a problem as long asI have a fast ADC running. (currently looking at the TI ADS1258). Doyou think that my ADC readings will be inaccurate if I have the ADC ona board separate from the sensors? Specifically - I need the sets ofsensors at right angles to each other - so there will be three boardsall orthogonal to each other - and I'm thinking just one of thoseboards would have the ADC on it. They would then be connected to eachother with right angle pin headers or something like that. The tracelength from the ADC to the sensor should be under 4cm.Thanks!-Mike

joseph2k · Answer

While it is doable, maintaining 12 or more bit accuracy across a connectoris increasingly problematic as bits increase. I would put ADCs on eachboard.  Designers choice though.--  JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens.    --Schiller

Mike Noone · Answer

Hmm in that case... the Analog SF7656: doesn't look too bad atall. Expensive part ($17 each for 1K parts) but the added simplicity ofthe part might make a whole lot of sense. I could put one on each boardand be done with it - no muxes, no worrying about signal degradationbetween parts and boards, no sample and hold chips. Just a shared SPIinterface between the three boards.-Mike

PeteS · Answer

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

vasile · Answer

Hi all,The problem here is not the sample&hold but the time required forreading the 12 holded values with a reasonable accuracy (as fastest aspossible after sampling is finished) using a multiplexed ADC.regards,VasileMike No

Adrian Jansen · Answer

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 netDesign Engineer         J & K Micro SystemsMicrocomputer solutions for industrial controlNote reply address is invalid, convert address above to machine form.

joseph2k · Answer

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

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

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