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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? 



Attachments:   







Report Date:   02/07/2001

Last Updated:   07/01/2004

Document ID:   266ARJCR






	     

                 

          
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