Multifunction DAQ

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What happens with E-Series card when the computer is off?

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A coworker of mine has observed something odd when the computer is turned off. He is using a PCI-MIO-16E-4 to measure the output of a 4-20mA sensor across a 250 Ohm resistor. He also has a DMM measuring this voltage. When the computer is on, the DMM reading is fine. The DMM reading agrees with the values obtained from a LabVIEW program using the DAQ card. However, when the computer power is turned off (and the DAQ card is still hooked up as to take a reading) the DMM's measurement drops from 3.7 to 3.4 Volts (for example).

It would seem that the card is no longer being a high-impedance device, as voltage measuring equipment should be.

Any ideas on what is causing this problem?

Thanks,
Melissa Niesen
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Melissa,

In reading the E-series user manual, I see that the analog input amplifier characteristics are as follows:

Normal powered on..........100 Gigaohm in parallel with 100 pF
Powered off................820 ohm min.

If this is indeed the case, you may be seeing that 820 ohms making a difference when the card is powered down by turning off the PC.

Mark
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Well, I calculated that it should be a little over 2000 Ohms, so that would be within spec for the amplifier.

Thanks!
Melissa
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