NI USB-6009 Problem: 1.4V measured at open input with DAQ Assistant

[johanneshoer]

I have just chekced it:

I connected potential deviders and what I measure is a mess:

Potential devider with two resistors between 5V and Ground. R1 = 100k always (at the 5V side), R2 variable

R2 = 1M    ==> Voltage should be 4.54 V, I've measured 3.4 V

R2 = 100k ==> Voltage should be 2.5V, measured 2.3 V

R2 = 10k  ==> Voltage should be 0.45 V, measured 0.50 V

So what "This behavior is normal and does not affect the measurement when a signal is connected"???

I'm really upset!

Message Edited by johanneshoer on 06-05-2009 08:58 AM

[johnsold]

The analog input impedance is specified as 144 kΩ.  What you measured is consistent with that.  The USB-6008 is not suitable for measuring signals with high source impedance.

I agree that this is rather low input impedance for a voltage measuring instrument, but at least NI does specify it.

If you need to measure a voltage with a high source impedance, you either need to buffer the signal with a low output impedance amplifier or select a different measuring instrument.

Did you read the specs before you bought the USB-6008?

Lynn 

[johanneshoer]

> Did you read the specs before you bought the USB-6008?

Of course I did not! Who reads specs? 🙂

Yes I did, but the 144 kOhms did not jump into my face.

I can work around that problem. The deviation is linear, and I can calibrate that.

Johannes