Increasing Your DMM Throughput
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To extract more speed from DMM-based systems, you may have reduced the number of tests, purchased several DMMs, or purchased very accurate DMMs and run them at much lower resolution. This article discusses how you optimize both measurement and system speed to achieve superior throughput without making such sacrifices.
Table of Contents
Introduction
Engineers and scientists struggle to maximize system throughput in both R&D laboratories and manufacturing environments. To extract more speed from DMM-based systems, you may have reduced the number of tests, purchased several DMMs, or purchased very accurate DMMs and run them at much lower resolution. Using the National Instruments PXI-4070 6 ½ -digit FlexDMM, NI switch modules, and NI LabVIEW software, you can achieve superior throughput without making such sacrifices.
DMM Optimization
By optimizing both measurement and system speed, you can easily improve your DMM-based system throughput. Measurement speed is how fast a DMM can take a measurement, and system speed includes how fast a DMM can scan with an external switch, change ranges (1 V to 10 V) and functions (VDC to VAC), and take measurements. Addressing both of these speeds can dramatically increase automated test system throughput.
Measurement Speed
Every application has different requirements. Some demand the highest level of accuracy, while others require maximum throughput. Consequently, DMMs have selectable features or parameters that you can adjust to your specific requirements. However, before you can make the appropriate tradeoffs, you must first understand how these features and parameters affect measurement speed and accuracy.
The highest resolution of a DMM, 6 ½-digits, always corresponds to its slowest measurement rate. As you reduce the resolution of your DMM, you achieve faster measurement rates. To achieve higher measurement speeds with respectable accuracy, you might purchase a 6 ½-digit DMM, but use it at 5 ½ digits.
The unique architecture of the NI FlexDMM takes full advantage of both LabVIEW and the high-speed PXI bus to deliver superior measurement performance. The FlexDMM offers a continuously variable DC reading rate from 5 S/s at 7 digits to 5 kS/s at 4 ½-digits, as shown in Figure 1. With the NI-DMM driver software, you can fine-tune the FlexDMM to meet your exact requirements:
- Autoranging measures a signal when you do not know which input range to select.
- Autozero removes the effects of several kinds of drift, such as temperature-related, long-term, and low-frequency drift.
- Offset-compensated ohms (OCO) eliminates the thermal offsets in low-level resistance measurements.
- ADC calibration, exclusive to the FlexDMM, removes long-term drift in the ADC by routinely calibrating the ADC back to a single well-controlled component.
- DC noise rejection (DCNR), also exclusive to the FlexDMM, suppresses the noise coupled onto a DC measurement. The FlexDMM includes three types of noise rejection -- normal, second-order, and high-order.
With LabVIEW, you can quickly adjust each parameter by simply adding a subVI or a property node, as shown in Figure 1.
Figure 1. FlexDMM Optimization Program
To further simplify the optimization process, you can use the NI-DMM driver software auto mode, which implements features only when appropriate.
System Speed
Every function change (DCV to 2-wire
) or range change within a function (1 V to 10 V) requires a DMM reconfiguration, and reconfiguration time can be on the order of a few milliseconds. As a result, reconfigurations, whether due to hardware or software latencies, can substantially slow down automated test applications. Still, you can easily increase system speed by optimizing the order of your measurements to minimize the number of reconfigurations.
Hardware Latencies
The types of relays used in traditional DMMs can cause hardware latencies. Each function change and sometimes range change requires switching an electromechanical relay inside a traditional DMM, and these relays have switching times of several milliseconds. The FlexDMM uses solid-state relays, which have switching times of less than one millisecond, to reduce this hardware latency. Even the random measurement sequence shown in Table 2 executes much faster with the solid-state relays on the FlexDMM. To further increase system speed, you can take similar measurements contiguously, as shown in the optimized measurement sequence in Table 1.
Software Latencies
For every reconfiguration cycle, you must open a session with the DMM, configure the DMM to the correct state, and close the session with DMM. However, that does not mean you have to open and close a session with the DMM each time you need a range or function change. Instead, you can implement all the range and function changes inside a LabVIEW loop structure, as shown in Figure 2. By making all of your measurements in a loop and closing the single session after you complete the measurement sequences, you can greatly reduce software latencies.
Figure 2. A LabVIEW Optimization Technique
Optimizing DMM and Switch Reading Rates
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