[Tinyos-help] ADC reading fluctuations

Michael Schippling schip at santafe.edu
Tue Nov 9 09:02:16 PST 2010

Presuming you are wired the way I said, the ADC input of the
micro-controller is now in parallel with your sensor. You can
use the {guessed,calculated,speced} input impedance to figure
out a more exact resistor divider. There are devices that have
much higher impedance where you might have to worry about the
input, but given that your sensor divider seems to be an order
of magnitude smaller than the input it probably doesn't make
much difference.

To guesstimate the input impedance, wire a variable resistor from
the Vref directly to the ADC input similar to what you had before.
In this case, the bottom half of your resistor divider would be
the input itself. Then you could twiddle the pot around until you
got a half-scale ADC reading. So the input impedance would be equal
to the resistor value.

good luck...

krisa wrote:
> A huge thank you!
> I Knew just connecting the variable resistor to adc pin and ground
> seemed like not the best choice, but didn't know about voltage divider
> (as I am quite new to electronics)
> With 10 kohm resistor in voltage divider it just worked (I got
> readings of approx 2000 while the variable resistor vas strainght
> (~10kohm) and approx 3700 when it was bent (~25kohm) So for this
> particular use it works well enough, but because I would also like to
> get a bit smarter - could you explain how the impedance of ADC input
> you mentioned affects the whole thing? And how do I measure it - with
> multimeter on ADC/ground pins?
> Also If I understand correctly - then simply connecting the ADC to
> ground should not have worked at all? And the voltage that I was
> reading - that came as a feedback from the ground wire then?
> Thanks again!
> On Tue, Nov 9, 2010 at 1:31 AM, Michael Schippling <schip at santafe.edu> wrote:
>> Less than an order of magnitude off...
>> Not bad for failing memory circuits.
>> I think the ATMEGA might have been around 250K.
>> Or maybe the 165f818 PIC...
>> MS
>> Yong, Chee Yeew wrote:
>>> Not being a lot of help here, but I measured / calculated the input
>>> impedance of the MDA300 ADC input recently, and it gave me 500kOhms.

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