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ADC 11 outputting voltage on inputs?

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ADC 11 outputting voltage on inputs?

Postby wizzardmr42 » Wed Nov 12, 2008 8:28 pm

Hi,

My ADC-11 seems to be outputting a volatage on any inputs that it is taking measurements on. If I connect a low resistance circuit, this isn't a problem, but I rigged up a couple of potential dividers in the mega-ohm range to scale down the output of a circuit where I need to measure a resistance against the current of the circuit (the voltage varies up to 20.9V). The problem is that the resistance of the resistor that I am measuring can go up very high as well, so I have used 1 and 8.2 mega-ohm resistors together as potential dividers to try to minimise the effect they have on the test resistor. (I am aware of the ADC's internal 2.5mega-ohm resistor). If I turn on the ADC with the power supply off, the ADC appears to be measuring around 0.49V. If I measure it with a multimeter, the multimeter reads around 0.148V and the ADC reading changes to match (I assume due to the internal resistance of the multimeter).

Does anyone have any way that I can get around this?

As a side point, I am also getting noise, but I am going to try to get hold of a capacitor to sort this out. I'm not sure what the max frequency of the noise is though - the ADC scope is showing up to 5kHz, but I don't know if it goes a lot higher and I have no other equipment I can test it with. Any ideas?

Thanks,

Martin
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Postby PeterF » Fri Dec 05, 2008 12:53 pm

Hi,
I am afraid that is little you can do about this unless you add a high impedance to low impedance buffer circuit to the input of the channel you use. The ADC-11 input will naturally float up to a value of about 1.7 to 1.8 volts when left open circuit. You can compensate for this by a bit of calculation when you take your readings but it is not simple!
Regards,
PeterF.
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