minimum and maximum voltage range for AI DAQmx (PCI - 6251)

[DSPGUY1]

Hi,

I have a genral question about the correct use of max. and min. voltage on AI during data acquisition. How can we set max and min voltage for acquisition. For example in one case we have a very small input signal in the range of hundered of microvolts and in another  case few of volts etc.

Please let me know if there is any good article written on this subject that can help me to understand quantization of analog signal related to NI DAQmx cards.

Thanks

AM 

[devchander]

Assuming that you are using LabVIEW with DAQmx

Simplest approach is use an individual 'create DAQmx virutal channel.vi' function for each channel and select the appropriate Min and Max value as shown in attached pic. ( Note: in single task , use channels on the same device and do not combine channels from 2 different DAQ cards)

Any doubts, do ask

Hope this helps,

Regards,

Dev

[DSPGUY1]

Hi,

My question is more general. What is the significance of setting voltage range. Its relationship to quantization. For example, if the received  analog signal is very small and I set voltage range from -10 to 10 then what would happened to digital signal that is generated by AI voltage vi. or if signal is in the range of +5 , -5 volts and I set voltage range in 0.1 to -0.1. etc. IS there any mathematical equation that describe the relationship between amplitude of analog signal to be digitized and the number of bits on card etc.

Thanks

AM

[Dennis_Knutson]

When you specify the min and max, the DAQmx driver will then automatically select the best range for that signal. Say for example, the board supports +/-.1, +/-1, +/-5, and +/- 10 volt ranges and it's a 12 bit board. If you enter max/min of +/- 2, then the board will be set to the +/-5 volt range. Your resolution is then 10 volts (total voltage swing) divided by 4096 (2**12). You can get the actual voltage range selected by using the DAQmx Channel properties AI.Rng.High and AI.Rng.Low.

If you specify a min and max that is less than the actual signal amplitude and it is greater than the actual range used, then you will clip your input signal. DAQmx does not do an autoscale.

[TheWoost]

Hi DSPGUY1,

Probably the best source of information to address your question is the M-Series manual that can be downloaded here.  The onboard NI programmable gain instrumentation amplifier (NI-PGIA) amplifies or attenuates the AI signal depending on the input range.  On pages 4-2 through 4-4 of the manual, there is information about the analog input range as well as the formula for determining the nominal resolution of a given input range.  The formula is as follows:  (max-min) / (2^16) = nominal resolution

A table is included for the NI 625x series for the most commonly used input ranges and the nominal resolution of each.

Regards,
Andrew W
National Instruments