Achievable Scan Rate with SC-2040

Hardware: Signal Conditioning Other>>SC Analog Accessories

Problem: How do I calculate the maximum scan rate that I can achieve with my SC-2040?

Solution: There is one basic equation that is applicable to all National Instruments Simultaneous Sampling devices that use track-and-hold technology. The equation below will yield the maximum scan rate that the system can achieve before accuracy is compromised.

Note: Warnings and Errors in NI-DAQ are not good indicators of safe or accurate scanning rates. Follow this equation to find the proper scan rate for your SC-2040 module.

Max Accurate Scan Rate = 1/(Track Time + (N-1)*Settling Time)

Here is a description of the variables in this equation:

N
Number of channels to be scanned
Track Time
Amount of time that the track and hold amplifier must be in track mode before it can be placed in hold mode and still get guaranteed accurate readings
Settling Time
This is the minimum time between scans and still guarantee accuracy. This can be limited by a couple of factors including the sample rate of the DAQ board, and the settling time of the multiplexer on the SC-2040 module. Once the minimum settling times are determined for the DAQ board, and the SC-2040 mux, the largest settling time is used in the equation because it is the limiting factor.

Here are the SC-2040 specifications necessary to calculate the achievable scan rate:
Track Time (bit dependent)= 7us (12-bit DAQ), 50us (16-bit DAQ)
Settling time (bit dependent) = 1us (12-bit DAQ), 2us (16-bit DAQ)

Here is an example calculation:
DAQ Board = PCI-MIO-16E-1 (12-bit)
Scan Rate = 1.25MS/s
Settling Time = 1 microsecond
Number of channels to be scanned on SC-2040 = 8

Maximum Accurate Scan Rate = 1/7us + (8-1)MAX(1us,1us) 
= 1/(7us + 7*1us) = 71,428 Scans/s


Related Links:   KB 2K2EACWD: What Parameters Do I Need to Calculate the Maximum Scanning Rate of a Simultaneous Sampling SCXI Module? 



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Report Date:   02/07/2001

Last Updated:   07/01/2004

Document ID:   266ARJCR






	     
                 
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