usb-6211 voltage change when connected

[tbd_ced]

We have a PCB with a voltage divider.  We are trying to read a battery voltage of 25 V.

Our voltage divider is 10:1 with a 90k resistor and a 10k resistor in series.

Before we attach the USB-6211, the voltage divider works correctly.  The voltage between the two resistors reads 2.5 v.

However, when we attach the USB-6211, the voltage between the resistors drops to 1.0 v, while the input voltage stays at the correct value of 25 v.

This occurs as soon as we connect the USB-6211 and it does not matter whether or not the USB-6211 is powered up by the PC or not.

The voltage divider is shown in the attached schematic.  The voltage signal comes in on the left on Pin 2 (it says 36 v on the diagram, but it is really only 25 v).  It immediately goes through the voltage divider, then goes out to a meter and to the USB-6211.

Any thoughts?

[AnalogKid2DigitalMan]

What two pins on the DAQ does the voltage and ground connection go to?

-AK2DM

[tbd_ced]

The ground goes to AI GND.  We put the positive signal voltage to AI 1.  However, we have tried it on all analog channels, AI 1 - AI 15.

[Leah-L]

When you measure this, do you have your input configuration set to RSE or differential? 

Have you attempted a differential voltage measurement using AI0 and AI8 for example?

[tbd_ced]

We are using RSE.  I have not tried the differential, but I will give it a try.  Thanks.

[AnalogKid2DigitalMan]

I don't think the mode (RSE, DIFF, etc) would be the root cause of the voltage dropping since the input imperdance of the AI channels is on the order of Gigohms.

I presume the external meter displays the drop when the DAQ is connected?

I suspect a ground loop- to test, have the external meter monitoring the voltage with the DAQ disconnected. Then connect the AI+ and look for a change in voltage. Follow that by disconnecting AI+ and connnect the DAQ ground while monitoring. If the meter only changes when only the ground is connected, you may have a ground loop in your wiring.

Other possibility- DAQ damaged? Cany you run the self-tests in MAX?

-AK2DM