Calculating the Maximum Achievable Scan Rate of Simultaneous Sampling SCXI Modules

Hardware: SCXI>>Modules>>SCXI-1140

Problem:
How do I calculate the maximum scan rate that I can achieve with my simultaneous sampling SCXI module?

Solution:
The SCXI-1520, SCXI-1530/1 and SCXI-1140 analog input modules are capable of simultaneous sample and hold (SS/H) signal conditioning. This feature provides multiplexing DAQ devices the ability to return synchronized samples of channels with negligible skew time between channels. SS/H signal conditioning is performed with track-and-hold circuitry.

The maximum scanning rate of a system is limited by the slowest component on the system. In a setup consisting of a data acquisition (DAQ) board and simultaneous sampling SCXI modules, the maximum scan rate that can be achieved is a function of the acquisition time of the track-and-hold circuitry and the safe interchannel period of both the SCXI module and the DAQ board.

Note: Warnings and errors generated by NI-DAQ, the driver of the DAQ board, indicate that the measurements taken previous to the generation of the error may not be accurate. NI-DAQ will generate an error or a warning when the maximum scan rate of the fastest component is exceeded.

For accurate measurements, you can use the following equations to calculate the maximum sampling rate when scanning SCXI systems with at least one SS/H module in your scan list.

NI-DAQmx

Maximum SS/H Sampling Rates (multiplexed mode)
sampling rate = 1 / [ (hold time) + ((no. of channels) - 1) * (greater of MSTm or MSTs) + (track time) ]

Maximum SS/H Sampling Rates (parallel mode)
sampling rate = 1 / [ (hold time) + ((no. of channels) - 1) * (MSTm) + (track time) ]

NI-DAQ Traditional

Maximum SS/H Sampling Rates (multiplexed mode)
sampling rate = 1 / [ (no. of channels) * { greater of [hold time] or [ sqrt((MSTm)^2 + (MSTs)^2)) ] } + (track time) ]

Maximum SS/H Sampling Rates (parallel mode)
sampling rate = 1 / [ (no. of channels) * (MSTm) + (track time) ]

Equation Definitions

Sampling rate	frequency of acquisition of all channels
Hold time	time between holding all the SS/H channels and the time the first AD conversion occurs (3us)
Number of channels	the total number of channels being sampled in the scan list (SS/H or not)
Minimum settling time of MIO	MSTm = 1 / (maximum sampling rate of MIO)
Minimum settling time of SCXI	MSTs = 1 / (maximum mux rate)
Track time	the minimum time between the last AD conversion of the current scan and the hold of the SS/H channels of the next scan (7us)

Device	Relative Accuracy	MSTs
E Series DAQ board (12-bit)	+/- 0.012%	3 us
E Series DAQ board (16-bit)	+/- 0.0015%	10 us
SCXI-1520 module	+/- 0.012%	3 us
SCXI-1530/1 module	+/- 0.012%	3 us
SCXI-1140 module	+/- 0.012%	3 us

For more "Settling Time to Full-Scale Step" values of DAQ boards, search for that characteristic in the specifications of the corresponding User Manual. DAQ manuals are available in our Product Manuals library.

Examples

Here are two example calculations based on the same setup. In the first calculation the SCXI-1520 is used in multiplexed mode using the Traditional NI-DAQ driver. In the second example the SCXI-1520 is used in parallel mode with the NI-DAQmx driver.

DAQ Board: PCI 6032E (16-bit)
Maximum scanning rate of DAQ board= 100 kSamples/s
Minimum Safe Interchannel time for a 0.0015% relative accuracy= 10 us (microseconds)

SCXI Module: SCXI-1520
Driver: Traditional NI-DAQ
Scanning Mode: Multiplexed Mode
Number of SCXI-1520 channels to be scanned = 8

Max Accurate Scan Rate = 1 / [ (no. of channels) * { greater of [hold time] or [ sqrt((MSTm)^2 + (MSTs)^2)) ] } + (track time) ], where

no. of channels = 8
hold time = 3us
MSTm = 1 / 100kS/s = 10u
MSTs = 10u
track time = 7us

Therefore, the maximum accurate scan rate is 12,500 Samples/s

SCXI Module: SCXI-1600 used with SCXI-1140
Driver: NI-DAQmx
Number of SCXI-1600 channels to be scanned = 8

Max Accurate Scan Rate = 1 / [ (hold time) + ((no. of channels) - 1) * (MSTm) + (track time) ], where

no. of channels = 8
hold time = 3us
MSTm = 5us or 10us (see explanation below)
track time (roughly corresponds to acquisition time from the 1140 user's manual, but you can assume 7us) = 7us

The maximum sample rate of the SCXI-1600 is 200kS/sec, so the MST = 5us, but if you want to settle to 16 bits of accuracy then the MST = 10us.

Therefore, the max sample rate when using all 8 channels of the 1140 would be 22kS/sec (this doesn't include any special regard for accuracy)

An NI-DAQ error will not be generated until a settling time violation has occurred with the DAQ device, regardless of whether the SCXI module is given ample time to settle to a predetermined accuracy. Between the maximum accurate scan rate and the scan rate when NI-DAQ generates an error, the acquisition time of the track-and-hold circuitry is held constant, while the scan interval is decreased.

Please reference the attached Excel spreadsheet for many more examples on calculating the maximum accurate scan rate for your SCXI system.

Related Links:
KnowledgeBase 2K2EACWD: What Parameters Do I Need to Calculate the Maximum Scanning Rate of a Simultaneous Sampling SCXI Module?
KnowledgeBase 27SDGMCM: SCXI Scan Rate
Product Manuals: Getting Started with SCXI
Product Manuals: SCXI-1520 User Manual and Specifications
Product Manuals: SCXI-1530/1 User Manual
Product Manuals: SCXI-1140 User Manual

Attachments:

Report Date: 04/02/2002
Last Updated: 10/20/2008
Document ID: 2K1BIKWD

Your Feedback!

Poor | Excellent

Yes

Document Quality?

Answered Your Question?

Please Contact NI for all product and support inquiries.