ADC-20 problems with high impedance signals

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ADC-20 problems with high impedance signals

Post by rc_campbell » Fri Apr 15, 2011 8:39 pm

Hi Everyone

I'm trying to measure several high impedance signals with an ADC-20 and it's doing things that I don't expect.

One signal is a 10K thermistor in an existing divider, 3V - 10K - 10K NTC - GND. The ADC-20 is connected to the center of the divider with 10K series resistance into the ADC-20, but it is changing the signal when connected, seemingly reflecting changes that occur on other ADC channels. What's up?

On an adjacent channel, I am measuring a voltage with a high impedance divider, 0-8V signal - 301K - 100K - GND, with the ADC-20 in the middle of the divider and configured for +-2.5V range. The documenation indicates that Radc is 1 MOhm, but what I see seems to be below 800K Ohms. Is this typical?

Are the channels measured in parallel or serially? Is this a Sigma-Delta converter that is directly connected to the pins and capacitively loading them with switching currents or is it buffered?


I'm getting excellet results with low impedance setups, but this isn't working at all well for these

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Re: ADC-20 problems with high impedance signals

Post by PeterF » Thu Apr 21, 2011 10:09 am

The ADC-20 is a single converter with an 8 way multiplexer front-end, hence the sample rate is divided by the number of channels used. This sounds like you are exceeding the "Common Mode" voltage rating at the inputs which will give strange results on all channels. Nothing to do with the actual input impedance.
Common-mode range
Channel to common (AG) :- ±1.9 V (±39 mV to ±1250 mV ranges)
Channel to common (AG):- ±3.0 V (±2500 mV range)
Common to earth ground (AG):- ±30 V
This means that, for instance, on a channel using the ±1250 mV range, no input voltage can be greater than ±1.9 V with respect to the common Analogue Ground (AG)connection. This may mean that AG needs to be taken to a voltage mid-way between the most positive and most negative input voltage.

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