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Topic: Differential ADC (Read 1 time) previous topic - next topic

pvthesis

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,


Grumpy_Mike

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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?

Anachrocomputer

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.

rocketgeek

#3
Apr 30, 2009, 05:52 am Last Edit: Apr 30, 2009, 06:23 am by rocketgeek Reason: 1
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.

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