4070 Ohms Test Voltage

[Odd_Modem]

I am using a PXI-4070 to make resistance measurements in a powered circuit which consists of resistive ladders and CMOS switches. Some measurements are unreliable and will return values centered on two peaks, such as 25 Ohms or 95 Ohms. If I sub a portable DMM such as a Fluke it returns a value such as 300 Ohms, which is close to the theoretical value. The circuit under test has a variety of protection diode shunts and it seems like the 4070 is turning on some of these diodes. When I run my 4070 with the soft front panel, and select either 2W or 4W resistance, the unloaded voltage on the terminals is always 12V. I see from the spec sheet that there are maximum test voltages for different ranges, but regardless of the range selected the open circuit voltage on mine is about 12V. Is my DMM defective? I am using auto-zero and offset compensation.

[Travis_W]

I tried the same thing on my 4070, but got slightly different results.  I used 2-wire mode and read around 12V for most of the ranges, but read below 5V for the 100MOhm range.  However, I'm not sure that reading the output voltage off the DMM with no load will give us the most pertinent results.

One thing to try would be to measure a few known resistors that are not powered.  If you get erroneous readings, then you may well have a defective DMM.  Until we can verify this behaviour, however, i would be more inclined to believe it is the powered CMOS network that is causing the problem.

Go ahead and measure a few resistors and let me know what you find.  If it is your DMM that is the problem, we can get you set up with a repair.

Thanks
Travis

[Odd_Modem]

The meter in question seems to be ok when it measures a DUT out of circuit. The CMOS switches are really just a resistive load, but they are biased for +/- 5V operation. When the meter is measuring a resistor, the impressed voltage falls in the range specified in the data sheet, but we are switching the meter terminals from point to point, and I think during the break action of the switching, the impressed voltage is popping back to 12V. These are just several of about 200 measurements I am making, and the meter performs like a champ for the other measurements.

[Travis_W]

Great!  Good to know your DMM is working properly.

Refer to this portion of the Resistance Measurements section of DMM Help to learn about Offset Compensated Ohms.  Hopefully your CMOS circuit will fall within the acceptable range of values for this procedure to be useful. 

To configure your code for this, refer to the NI-DMM help section about the Offset Compensated Ohms property.

Thanks.  Let me know if you have any more questions.