signal level scaling and offset

[Bailey7862]

I have a 0 to 4.5 VDC non referenced signal that I would like to accurately measure. I have this connected to a PXI-6225 DAQ board in differential mode with the range set for -5 to 5 VDC. With this configuration the system is only able to use ½ of the DAQ boards range. Do you have hardware to offset and scale my signal for a -4.5 to 4.5 VDC input to the ADC? I have used “old” E series 16 bit DAQ boards with a 0 to 5 VDC range that were able to measure my signal to a standard more accurately than I can now do with my 6225 board. I believe the issue is with the difference in range used. The 6225 board has double the bin size compared to my board with a 0 to 5 volt range.

[Efrain_G]

Hi there,

Just like you mention the range on the 6225 can only be set to values that are valid according to the ADC. All other values will be coerced to the closest valid ones. Fro example, if you try to set your range to go from -4.5 to 4.5, it will be coerced to -5 to 5. However, when you use the -5 to 5 range you are still using the full range of your ADC. In fact, any valid range that you choose will be automatically configured to use the full resolution of your ADC respectively. 

As far as your hardware question, unfortunatley we do not offer hardware that will offset and scale your signal to -4.5 to 4.5 VDC. Nonetheless, you can easily implement this yourself using external circuitry such as a voltage divider.

[Buechsenschuetz]

Your problem can be solved with a single operational amplifier with a gain of 2 for the signal path and adding a reference voltage of suitable value. You will find the basics about such a circuit in any textbook about operational amplifiers. Of course you will also need a (symmetrical) supply voltage for this circuit.

[Brad_K]

Hi Bailey7862,

Which E Series board are you comparing the NI 6225 to, and which accuracy specifications are you comparing? Adding a bit does not necessarily double the accuracy.

Brad