ADC Are differential voltages always sampled simultaneously?

olhexy

Hi,

I would like to measure the inductivity of a coil with mega1284P. It has to be fast (~0.2 msec) and 'accurate'.

As far as I understand: the samples for a differential ADC measurement are taken for both channels at the same time.

But I would like to take 2nd channel sample a short time after 1st channel, and then treat them as a differential voltage measurement.
Do I want the impossible, or is there a trick?

(I am afraid I need some kind of extra 'sample and hold' for that...)

(BASCOM-AVR version : 2.0.7.7 )

Meister · Joined: 27 May 2010 Posts: 319

AdrianJ · Joined: 16 Jan 2006 Posts: 2483 Location: Queensland

A differential measurement using the ADC MUX does exactly what it says, samples the difference in level between two channels. Not by measuring each channel separately, and than subtracting, but by differencing ( in analog terms ) the two channels, then measuring the result with the ADC. So it makes no sense to ask to delay the channels in this way.

But that said, in processor terms, 0.2 msec is a long time. You can get up around 50-100 ksamples per sec on a standard Atmega ADC. So just sample ( differentially if necessary ) once, wait 200 us, and then sample again. Then use the two readings. Sending a pulse on an unrelated line on each sample, and scoping that line to measure the actual time between samples will give you an idea of the timing, and how long to wait to get the interval you want.

Something like:

olhexy · Posted: Tue Feb 25, 2014 4:13 pm Post subject:

Meister and Adrian, thanks for your replies.

Adrian, it was my first idea to do repetative samples at 5 kHz. But the problem is that within such a short time the voltage differences are minimal in relation to the full scale. So there is almost no resolution left.
A differntial sample is easily amplified 10x or 200x.

AdrianJ · Joined: 16 Jan 2006 Posts: 2483 Location: Queensland

I see your point. A sample and hold is another way, as you suggest. Another would be to use a much higher resolution ADC, like a Delta-Sigma converter. But beware the sampling speed, and the system noise !

You say you want to measure the inductance of a coil, presumably by applying a constant current, and measuring the voltage at two times. Any other special requirements ? Or can you afford just to wait longer, hence getting a bigger voltage difference.

olhexy · Posted: Wed Feb 26, 2014 2:00 pm Post subject:

(I am playing with a solenoid. Main inspiration is from this document: http://158.132.178.85/norbert/Papers/C005.pdf The plunger position has to be found by measuring the systems changing selfinduction.)

Adrian, I am trying to avoid a complex S&H.

Found this: http://memo.cgu.edu.tw/wu-shiung/course/CommunIC/9__Sample_and_Hold_4_24.pdf

My idea: 2 equivalent S&H circuits with just a BS170 and a capacitor like shown at page 9 ("Fully different sampling" - slide 33). This looks very simple. The difference is that each FET has to be separately switched: second FET a short time dt after the first FET.
I could give some microseconds settling time before picking up the voltages by AVR's ADC, and then get a pretty good differential voltage measurement result, I think. Because both S&H are the same the combined errors are widely eliminated.
Your thoughts?

AdrianJ · Joined: 16 Jan 2006 Posts: 2483 Location: Queensland

Firstly, we should probably move this discussion to a different forum, like "Various". At this stage it has not much to do with Bascom.

Moved to:
http://www.mcselec.com/index2.php?option=com_forum&Itemid=59&page=viewtopic&p=63515#63515

kimmi · Joined: 24 Feb 2006 Posts: 1922 Location: Denmark

topic moved to Various