Test and Measurement Forum

ADC 11 outputting voltage on inputs?

Post any questions you may have about our current range of USB data loggers

ADC 11 outputting voltage on inputs?

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


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?


Posts: 1
Joined: Wed Nov 12, 2008 8:19 pm

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

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!
Advanced User
Advanced User
Posts: 435
Joined: Thu Jun 07, 2007 10:53 am
Location: Cambridgeshire

Return to USB PC Data Loggers

Who is online

Users browsing this forum: No registered users and 0 guests