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?