Inaccurate Measurements when measuring current and coltage using PCIe 6259

[SKauth]

Hi there,

very simple setup but still strange results. What I'm trying to do is measuring supply current and voltage of an electronic device using a NI PCIe 6259 card and a 10 Ohm shunt. But I receive slightly inaccurate results.

I simplified the setup to track down errors, so I begin with what's left: A 3V battery pack with a differential AI connected to the poles to measure current and a shunt leaving from the positive connector and open on the other end. This shunt is connected to another AI pair.

So as there is no load connected and the DAC input currents are supposed to be neglectable (10G Ohm), I would expect a reading close to zero. Actually I get after averaging a DC value of 14µA, which seems quite OK considering a noise voltage of 50µA

BUT: I discovered that when I disconnect the voltage measurement, the current drops 5µA! Also when I connect a multimeter in the wires for voltage measurement, I get 50µA flowing into the DAC, which gives 60kOhm at 3V. Another strange thing is that the current into the card is rising with sample rate (the mentioned values are for 400kS) and input voltage. And when I remove the current measuring wires, the current into the card goes to zero.

It seems to me that there is an issue concerning channel-to-channel separation. But I couldn't find any information on what this value should be. I know that a 5µA error seems not much regarding a 100mA Range, but still, why the high input current? And being able to measure such low currents would really be nice in my application.

I tried to change channels, use the other connector, both connectors, disconnected the SCC-68 board and changed to RSE. The latter reduced the error by 50%, but still this is too much. And I need differential when using grounded voltage sources.

Is this behaviour  known? Is there a solution? Do I need a repair?

Regards,

Simo

[SKauth]

Ah, and what I forgot to mention:

The current flowing into the DAC stops when I stop the measurement.

[StefanA]

Simo,

I have problems to understand your wiring and measurement. What current is your battery pack producing? How do you wire the battery and the shunt resistor to the AI?

Attachea post concerning the PCI-6259 and low current measurement.

Measure low current (micro-Amp) using PCI-6259

http://forums.ni.com/ni/board/message?board.id=250&message.id=33944&requireLogin=False

Stefan

[SKauth]

 Here's a quick sketch about how I connected the shunt. A load was not connected.

Actually I'm not so much concerned about the  accuracy of the setup but about the fact that the channels interfere with each other and produce a relatively high input current.

Thank you for the link to the other thread, we might consider buing a DMM, but actually I don't see, why the 6259 should not be suitable for my purpose. As long as I don't measure both channels, it works fine.

So far it seems to me that the input multiplexer is responsible for this effect. But it would be nice to hear from somebody else about this to be sure that the card is not broken.