Difference Between High-Resolution and High-Speed Mode in C-Series Modules

Issue Details

Many C-series analog input modules have two modes of operation, namely:

1. High Speed Mode
2. High Resolution Mode

I understand that they offer different conversion times, but what exactly is happening differently inside the module in either case?

I want to use my C-series module in High-Speed Mode and then filter it the way High-Resolution mode would, so I can optimize my signal for speed and resolution. Is this possible to do this?

Solution

The C-series modules do indeed offer different conversion times based on the ADC mode of operation. This also impacts the noise in the signal as high-speed mode does not filter for noise, while high-resolution mode effectively eliminates 50/60 Hz line noise.

The primary difference in their operation is in the period of time the ADC spends on processing the signal.

• High-Speed mode: The ADC oversamples and digitally filters the input signal over a small period of time.

• High-Resolution mode: The ADC oversamples and digitally filters the input signal over a much longer period of time. This is usually long enough to filter out both 50/60 Hz noise

You can certainly acquire data at the maximum rate in High-Speed mode and then implement your own filtering in the host to increase resolution. However, the results won't be exactly the same as what the module does internally for High-Resolution mode. In particular, these results will be different because they won't be continuously oversampling and averaging over the entire time since every new sample in High-Speed mode will reset the ADC filtering process and start over. 

Additional Information

C Series modules that support the ADC timing mode properties in DAQmx have delta-sigma ADCs that support configuring the ADC's digital filter in order to trade off speed vs. resolution and/or reject specific frequencies (such as 50 or 60 Hz).