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Hi everyone,

I have a pyranometer with differential output. I must read this voltage.
I have read that the minimum input impedance for this sensor is 1Mohm. Can someone confirm that the input impedance for AVR ADC is 11Mohm ? How to use AVR ADC in differencial mode ?

Thannks in advance

Best regards,

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Can someone confirm that the input impedance for AVR ADC is 11Mohm

No sorry it is only about 10K

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How to use AVR ADC in differencial mode ?

It hasn't got one, you could measure the two outputs sepratly and subtract the difference.

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I have read that the minimum input impedance for this sensor is 1Mohm.

Are you sure that is a high output impedance for a sensor. Your only hope would be to put it through an op-amp to buffer it.
What sort of voltage does it produce? What resolution do you want the readings?
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For this sort of application, you may need an "instrumentation amplifier".  Google for it to find out more, or try this:

http://en.wikipedia.org/wiki/Instrumentation_amplifier

Analog Devices make one called the AD620, which has been mentioned before on these forums -- might be worth searching.
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Analog Devices has a nice instrumentation amp called the AD620. It uses one external resistor to set the gain and that's the only external component. One caveat tho -- it is in no way a rail to rail part -- the closest it can get to the rails is about 1.2V. So, if you want to use the full range of the ADC, you'll need a dual. That's not too hard to arrange with something like a MAX680, which uses four capacitors and some internal switching magic to make +/-10V out of +5V input. The only caveat is that it's a bit noisy. The power supply rejection on the AD620 is at least 80 dB, so it's not that big a deal, but you should still make sure that the supply is well decoupled.

Editted to fix the output voltage of the MAX680, because I am a putz.
« Last Edit: April 29, 2009, 11:23:56 pm by rocketgeek » Logged

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