USB-6008 analog input measure of resistor divider failing

[richer]

Hello

I tried to measure the voltage of the resistor R1 from resistor divider,as you can see from the picture ,obviously,the voltage should be about 1.5V.however,the voltage what I got is very strange.The voltage is swing and can even reach several volt .It does not make sense.Here the 3V signal is given by a DC power supply.I also tried to measure the DC power supply directly and the voltage is right.

Anyone can explain it?

Thank you very much!

[richer]

the picture

[AnalogKid2DigitalMan · Accepted solution]

Input impedance for the 6008 analog inputs is a measly 144kohms shown in the link below which is affecting your divider ratio.

You will probably need buffer circuity with a higher input impedance between your diver and 6008, or use a different DAQ with higher impedance.

http://www.ni.com/pdf/manuals/375295c.pdf

-AK2DM

[Henrik_Volkers · Accepted solution]

In addition to AK2DM:

Without a buffer amplifier, but some more math you can find the value of R1. You have to expand the network with the 6008 input resistor network....

For certain ranges of R1 and increased uncertainty you can directly connect R1 and calculate the value 😉  (This only works for 6008 type inputs, have a look at the input circuit)

[richer]

Thank you very much!  I will have a try.

[richer]

Thank you very much!  I will have a try.