NI 9219 signal drift in 2-wire resistance mode

[nssid]

Lynn,

If the max wiper current (or recommended current) is 1 uA, why is the max permissible applied voltage given as 42 V ? The resistance of the pot is 5 kohm which means that the wiper current when 42 V is appplied is 42/5000 = 0.0084 amps (or) 8400 uA. Did I miss something here ?

I could get access to a temperature control room and I used the 9219 to measure the resistance of a spool of wire and there is no drift in the data. This is the result I got

The test was done at 20°C and the initial drop of resistance could be just the wire coming down from room temperature to 20 C.

[johnsold]

The key point on the wiper current is that the manufacturer recommends that you do not draw any.  The 8.4 mA is not wiper current.  That is the current through the body of the potentiometer.  By connecting to the wiper only circuits which draw essentially no current, no errors are introduced at the wiper contact point.  In addition to the 1 uA specification the paragraph to the right on the data sheet recommends an op amp buffer connected to the wiper.

In other words, they recommend MEASURING the wiper voltage with an ideal voltmeter (the op amp buffer) but not using that voltage directly to do any work.

I agree that your controlled temperature measurement suggests that the instrument is not the problem.

Lynn

[nssid]

I think that explains a lot. Thanks.

So, do you think I can use the NI 9263 to supply voltage accross 1 and 3 (refer the fig below) and measure the output voltage across 1 and 2 with the NI 9219 ?

[AnalogKid2DigitalMan]

Yes, that arrange could work if you observe the following:

The 9263 has a max of 1mA of drive current. This means you could use up to 5V across the 5000 ohm pot (5V/5000ohm = 0.001Amp)

However the 9219 in voltage mode has an input impedance of 1Mohm for the 60, 14 and 4 V ranges, not advisable since this could draw more than 1uA from the wiper, depending on the voltage apllied across the pot.

The 1 and 0.125 V ranges in voltage mode have an input impedance >1Gigohm. Very little current will be drawn through the wiper.

Thus I suggest you apply up to 1V with the 9263 and use the 1V range on the 9219 for maximum resolution.

You have not stated your needed accuracy or short/long term drift budget, so you may want to look at the specs of the cards for this.

-AK2DM

[nssid]

Thanks a lot ... I will look into the specs of the cards for the drifts and accuracy.

[johnsold]

AK2DM said exactly what I would have said if he had not already said it.

Lynn

[AnalogKid2DigitalMan]

Lynn:

Sometimes I find myself thinking the same about some of your posts. Scary 🙂

Respectfully,

-AK2DM