The Mote is running on about 3v and the ADC is DC coupled,
therefore it can convert input values between 0 and +3 volts
(or thereabouts). Anything higher is out of range, and anything
lower is possibly dangerous to the input. You should bias your
sample signal up by +1v so it swings from 0 to +2. If you are
dealing with a high enough frequency you can put the bias on
the ADC input and capacitor couple the signal. If you start
needing multi-farad capacitors you might want to explore the
opamp signal conditioning option...
Speaking for the Micas, the 10 bit values are unsigned from
0 to 1024. An arbitrary point in the middle...say a 1.5v bias
can be selected as a "zero" reading (if it all works out nice
the bias with no other input should give you a 512 value).
You then subtract that value from all the unsigned readings
and cast it to get a nice signed AC sequence.
I expect the 12 bit convertor (on the Telos?) is the same thing
with more bits.
MS
Hemanth H wrote:
When I tried out the oscilloscope application (Modified to sample an
external ADC input) and gave a sign wave as input (max +1V min -1V), I
am getting only a clipped sign wave in the oscilloscope application i.e
I am not able to see the negative part. The oscilloscope is not plotting
anything below 0. I checked the oscilloscope application (Java app
running on the comp)and the program running on the mote. I felt that the
oscilloscope application (Java app running on the comp)is fine.
I have the following questions regarding the ADC sampling app running on
the mote.
1 The ADC returns a 12 bit digital output, is it signed or unsigned?
2. If it is signed, how to get the negative value?
regards
hemanth
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