06-05-2009 08:53 AM - edited 06-05-2009 08:58 AM
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!
06-05-2009 09:40 AM
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
06-05-2009 09:56 AM
> 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