Archived: NI PXI-4070 - DC Voltage - DMM/Switch Handshaking Scan Rates
Updated Feb 19, 2020
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Benchmarking scan rate data is very useful when deciding what which devices are needed to meet system requirements. This is true of digital multimeter (DMM) and switch systems since scan rates are dependent on the speed of the digital multimeter, the switch, and the triggers between them. This document contains data for the typical scan rates that can be achieved utilizing handshaking.
There are many factors which will affect the overall scanning speed of a DMM/switch system. The major factors include the type of the measurement, the range of the measurement, and resolution of the measurement. The effect of these and other factors related to the DMM can be better understood by considering the digital multimeter measurement cycle. In addition to these factors, relay operate times, DMM settling times, and switch settling times limit the overall speed of the system. The following benchmarks provide a good starting point to see typical scan rates for common use cases.
The benchmarking application that was used to gather the following data is attached to this document. There are two top level VIs within the library. The DC Voltage with Datalog VI uses the Microsoft Report Generation Toolkit to log the acquired scan rates to an MS Excel Spreadsheet. The DC Voltage VI preforms the exact same scan rate test, but only displays the results rather than logging them to a file. Both top level VIs and all required subVIs are contained in the .llb file.
The NI-DAQmx API was used to program the switches in order to obtain these benchmarks. This program systematically cycles through each DMM resolution and range and logs all the resulting scan rates to a spreadsheet. The attached program is configured to make DC Voltage measurements, but can easily be used to benchmark scan rates for other measurement types and other devices.
The following benchmarks show typical scan rates when using handshaking. The DMM used for testing was a NI PXI-4070 set to measure DC Voltage. Both the measurement resolution and range were varied to illustrate the resulting effect on the scan rate.
There are ways to increase the speed of the system by adjusting the reading rate of a DMM. Depending on the requirements of the measurement, reducing parameters such as aperture time and turning off settings such as AutoZero will increase the overall scan rate of the system but may reduce measurement precision. These values can be experimented with to find a good balance between precision and speed for the measurement system. These benchmarks use default settings for switch and DMM configurations, however to ensure specified accuracy, extra settling time may be required.