resistance measurement with USB-6009

[cartonn30gel]

I'm trying to measure the resistance of a photoresistor by using the USB-6009. There is a 'resistance' option in DAQ assistant but it doesn't seem to display the right values. Here is what I'm doing:

Connections: GND------photoresistor------ai0

I'm really not sure if this is right but I assumed it could measure resistance just like a multimeter. I didn't try doing a voltage divider and using ohms law.

DAQ Assistant Settings: I 'add channel' by using the blue plus button, and choose 'resistance'. Then I choose ai0 under USB-6009. I adjust max and min values and read it continuously. First problem, the reading is usually negative and it flickers a lot. I read about -1.3k when I do this with a regular 10k resistor (not a photoresistor)

Obviously there is something wrong, but I'm very new to all this and can't figure it out by myself. Any help would be highly appreciated.

Thanks!

[Dennis_Knutson · Accepted solution]

The 6009 can not measure resistance like a multimeter unless you are proving a current source like the default assistant is setup for. By itself, it can only measure voltage. So, use a voltage divider.

[Adil_Rehman]

I have used voltage divider circuit to find out an unknown resistance. However, it still fluctuates and not giving a correct ans. Let me know any solution regarding this problem.

Thanks in advance.

[Henrik_Volkers]

Well, the 6009 input circuit is 'tricky', see pic from manual. Resulting in an input impedance of ~144kOhm.   If you measure with a voltage divider with a 6009 you should add a voltage buffer (one OP amp and a buffer cap) or use some math to compensate the input impedance.

However you can connect a resistor between AI and GND  (or AI and +5V) and measure the resistance 🙂    the rest is analog circuit math 😄  ..  including the possible resolution and resistor range  😉     . So it is possible to measure (certain) resistors with the 6009 without other circuitry ... but it's not the intended use and not supported by the driver.