trim

[lam lugano]

Hi!

I have a ni usb-6008 and i want to know the resistor of a trim with this board.
I have connected the trim with a resitor (R1= 570000 ohm), "in line" (i don't know how to say it), and i measure the voltage (v2) between the trim and the resistor with an analogical port of the usb-6008.
For calculating the resistor of the trim, i use the formula  "(R1*VOLtot)/V2)-R1 = Rtrim"  (voltot = 4.5 volt).

The results i read on the computer screen are complitely different from the teorical results obtained with a table.

i suppose it is the usb-board which does not work, but i don't know why.

Help!!!!!!!!!!!!!

[Chaos]

I think your diagram has made one faulty assumption.

You should not assume that V2 is 4.5 volts.  As a matter of fact the only way this can be true is if the trim is set to the same value as R1 (560k) thus splitting the total 9 volts into two equal halves.  I would instead measure the voltage at V2 and divide by the known resistance 560k to get a value for current.  The remainer of the 9 volts will be across the trim and if you divide this remainer voltage by the current you can calculate its resistance.

i = MeasuredV / 560k

Vt = 9 - MeasuredV

R1 = Vt / i

[BrainBurner]

Hallo!
I have the same problem, and i understand lam lugano very well!
something doesn't work with the usb device!
if i understand the attachment he has a total voltage of 4.5, but the difference is between 9 and 4.5 and not between 0 and 4.5, as usual!

Your way of resistor's calculation is the same than lam luganos's one!

If somebody had this problem and solved it, "can you reply?!"

When i take my measure with a multimeter and without using the usb device all work fine, but when i connect the device all goes wrong!

[Dennis_Knutson]

There's nothing wrong with Chaos's formula. What exactly do you mean by "all goes wrong"? How do you have the DAQ board wired and what configuration are you using (diff or SRE)?

[BrainBurner]

I never said that Chaos's formul awas wrong, I used it too. I only said that it's the same that I use!
By the way, I use a SRE because I need lots of analog input, so I have the same ground for all the input!

"All goes wrong" meens that the formula returns a resistance much highter from the maximum value of the trim (500k)!

I cannot understand the reason, but when I connect the circuit at the usb device the voltage decrease!

Do you have some idea?

[tuned99]

Without the 6008 connected, what is the measured voltage using the multimeter?  Then, with the same multimeter, what is the same measured voltage with the 6008 connected?  Can you also measure the impedance of the 6008 channel?

[AnalogKid2DigitalMan]

It's a no brainer if you look at the specs for the 6008's analog inputs- their input impedance is a measly 144kohm. So when you connect your 6008 to the 560kohm resistor, this parallel combination results in 114.5kohm. No mystery why results do not match expectations.

You need a high input impedance buffer between your 6008 and the circuit. OR a good quality DAQ card. You get what you pay for.