Greetings All,
I have a question regarding the 51 Connector MICAz Expansion
Connector. I already have a sensor built which outputs an
analog AC voltage measurement with a "maximum(peak)" amplitude
of about 2 volts AC. The amplitudes are not strictly 2 volts
in amplitude. The amplitudes can be anywhere between 0 and 2
volts AC. I am looking to digitize this waveform with the A/D
converter provided by the 51-pin expansion connector in the
MICAz mote. The issue however is that the output of the sensor
consists of just two wire leads (+) and (-)--"the ground". Is
there anyway that I could possibly get my sensor to work with
the expansion connector provided with the MICAz mote? My
overall objective is to digitize the waveform and have this
information sent to the PC and reconstruct the waveform. If
you could provide me with some insight regarding this I would
greatly appreciate it.
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Below is the response that I received from Crossbow's Technical Support.
It seems as though this might be correct, however I need to know if the
non-inverting amplifier that I need to build should be AC or if it should
be a regular DC non-inverting amplifier. The difference between the two
different types "Non-Inverting DC Amplifier" and "Non-Inverting AC
Amplifier" is provided here on pages 1 and 2 respectively:
www.national.com/an/AN/AN-31.pdf. If someone can provide me with some
feedback, I would greatly appreciate it. Thank you all once again.
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The 51 pin connector has ADC channels to the Atmel processor ADC[0..7].
They are 10 bit (probably 8.5 bit effective), and can be set to read in
one-shot mode or continuously. They are supported in both Moteworks and
Tinyos in many applications. The inputs are scaled for a range of 0V to
Vbattery, which is usually between 3.2 and 2.7 volts. The only issue is
if the AC waveform is centered on ground (does it swing negative?). If
it has an average value centered around 0V, then it will be necessary to
"lift" the waveform by giving it a dc offset of Vbattery/2. This will
ensure the entire waveform is within the capture range. Any unity gain
stable op-amp can do this with a simple voltage divider in the feedback
circuit. Apply the output of the op-amp to the ADC input, and use one
of the standard data sampling apps as a basis for readings.
(Moteworks\apps\tutorials\lesson_2)
-C.B.
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