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Topic: adc / accuracy? (Read 1 time) previous topic - next topic

RIDDICK

#5
Oct 05, 2009, 04:56 pm Last Edit: Oct 06, 2009, 03:20 am by RIDDICK Reason: 1
now i found the cause, i think:  ::)

when i measure the voltage with my arduino-oscilloscope
(it can do a sample rate of 7kHz to 8kHz at 500kBaud)
at the end of the 10m-line,
i get a cloud of values (no 50Hz wave, but much higher frequency)...

that cloud averages to the right value...

but when i add a capacitor
(i tried a red box-shaped plastic-foil-capacitor with 6.8uF, too),
i get 20mV less...

why is that?
i thought, capacitors would be nice "averagers"...   ::)

should i better ask in an electronics forum,
since this is surely not arduino related
(the arduino measures fine)?
-Arne

pwillard

#6
Oct 05, 2009, 06:20 pm Last Edit: Oct 05, 2009, 06:29 pm by pwillard Reason: 1
With every component that you add to the path between the sensor and the measuring point, it makes sense that you would lose some signal.  I mean, anything that does not AMPLIFY a signal... will weaken it.

Why not amplify the signal before you do things like send it through a MUX or a long cable.  You could use a a single supply op amp like a TLC272 with a 100K in the negative feedback path (with the - input also tied to 5V via 10K resistor)  Use a similar pair of resistors for the sensor input (From the + pin, use 10 K to LM35 source, 100K to GND) and use no more than .1uF on the signal wire  from the LM35 to GND.

You would then have a x10 Amplifier.  (I'm loosely basing this on an instrumentation amp configuration minus the extra input amps.  http://www.national.com/an/AN/AN-31.pdf)  I've used a similar circuit while testing a Motorola Pressure sensor.

pw

Grumpy_Mike

Quote
i get a cloud of values


That means something is oscillating. The capacitor stops it oscillating. That's why oscilloscopes are always the best way of measuring voltages in an electronic circuit. Have you got any decoupling across your multiplexer chip?
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

RIDDICK

yup - i put some 470nF caps and some 22nF caps in parallel to the supply voltage of the MUXes... i read ur article before...  ;)

but why doesnt the capacitor show the average value, when it is oscillating? is it the ESR of the cap that wastes the energy? but the ESR of a ceramic cap is below 15mOhm according to wikipedia...
when it is not oscillating, the capacitor doesnt loose so much voltage...

i was under the impression, that a capacitor would build the average as if it is an "analog average computing device"...

my plan is now to remove the cap from the MUX output and to make an "ideal capacitor" in software (by computing the average digitally from some thousand samples)...
is that a good plan?
-Arne

Grumpy_Mike

Quote
is that a good plan?

Not if it is still oscillating. You need to stop the oscillation before doing anything else.
This is because an oscillating input will not give you any sort of true reading for the value. You might not even have a problem.

Don't believe all you read on wikipedia.

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