The NI PXI-4070 FlexDMM™
A Benchmark Comparison of 6 1/2 Digit DMMs
Introduction
A test system is only as fast as its slowest component is a simple conclusion that
can be drawn from the popular idiom – a chain is as strong as its weakest link.
Taken in context, the slowest component of most automated measurement
systems is a digital multimeter (DMM). For years, test engineers have employed
numerous strategies to extract more speed from these devices in R&D
laboratories and on the manufacturing floor. Optimization techniques have
included cutting down the number of tests, reducing the accuracy requirements,
purchasing multiple DMMs, or even purchasing a more expensive 8 1/2 digit
DMM and running it at much lower resolution. These techniques can improve the
overall test throughput of a DMM, but typically at the expense of accuracy,
system cost, size, or all. The new National Instruments PXI-4070 6 1/2 digit
FlexDMM eliminates the need to use such techniques by providing the following
advantages over traditional DMMs:
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It has a fast A/D converter with a sample rate of 1.8 MS/s, so it can take
measurements more frequently
It uses the 33 MHz PCI bus, which provides transfer rates at least 100
times faster then GPIB interface
It takes advantage of high-speed host PC processing, which improves
system throughput every time the CPU is upgraded
The streamlined NI-DMM driver software offers tight integration with
LabVIEW
It executes rms-to-DC conversion, traditionally an analog problem, in the
digital domain, decreasing overall measurement time
It dramatically improves speed and accuracy of function and range
changes by using solid-state relays
Seamlessly integrates with National Instruments PXI and SCXI switches
by using the high-speed PXI trigger bus
This paper quantifies these advantages by benchmarking the NI PXI-4070
FlexDMM with two of the electronic industry’s most popular traditional 6 1/2 digit
DMMs. In this paper, these DMMs will be designated DMM1 and DMM2. The
FlexDMM will be tested against DMM1 and DMM2 in three typical measurement
scenarios – single-point, multipoint, and multifunction. These benchmarks
demonstrate that the FlexDMM is ideal for use in automated tests on both the
production floor and in R&D environment.
Test Setup
Measurement speed and system speed are good indicators of the performance
of a DMM. Measurement speed is how fast a DMM can take a measurement.
System speed is how fast a DMM can scan with an external switch, change
ranges (1 V to 10 V) and functions (VDC to VAC), in addition to taking
measurements. A typical automated test system includes both a DMM and an
external switch, because users can expand the channel count of their DMM with
a switch, without prohibitive expense.
Each system-speed test time includes the time it takes the DMM to hardware
handshake with an external switch. Each test was run with a National
Instruments SCXI-1128 24-channel solid-state multiplexer. This switch was
chosen because of its fast switching capability.
Various tests were run on each of the DMMs to benchmark the speed of singlepoint
measurements,
multipoint
measurements,
and
multifunction
measurements. To limit the number of tests, only the three most popular
functions were used – VDC, VAC, and 4-wire resistance. Other functions
produced similar results. The test system included the following hardware and
software:
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NI PXI-1000B Chassis
NI PXI-8176 embedded controller with:
o 1.26 GHz Pentium III processor
o 256 MB of RAM
o Windows 2000
NI LabVIEW 6.1
IVI-compliant DMM-specific instrument drivers
The PXI-4070 FlexDMM was housed in the PXI-1000B chassis and controlled by
the PXI-8176 controller through the PXI backplane. All tests were automated
using LabVIEW. DMM1 and DMM2 communicate to the PXI-8176 controller
through GPIB. Figure 1 shows how the following tests were connected.
Figure 1: Automated Test Configuration
Each function was tested to 6 ½, 5 ½, and 4 ½-digits of resolution, except VAC,
which was tested at 6 ½ digit resolution. This was done to get the maximum
performance out of all AC measurements taken and to limit the number of tests
performed. The front panel displays on DMM1 and DMM2 were disabled for all
tests) as was autozero for all three DMMs.
All measurement speed tests are in readings per second, which only includes the
data acquisition time. The system speed tests are in “effective readings per
second”, which describes the total number of measurement points returned from
the DMM divided by the total measurement time. Total measurement time
includes the initialization, configuration and data acquisition time for all the
functions (VDC, VAC, etc.) tested. For example, in a test where 450 points of
each type were measured:
450 (VDC) + 450 (VAC) + 450 (4-w Ω)
50 seconds (to acquire all 1350 points)
1350/50 = 27 effective readings per second
Single-Point Measurements
This automated test measures the time for the DMM to initialize, configure,
switch, take one measurement and transfer the data. The DMMs were configured
to perform two-way hardware handshaking.
RESULTS
Measurement Speed:
Single-point Readings per second in 10V Range, no sw itch
120
Readings per second
100
80
60
40
20
0
4½
5½
6½
Digits of Resolution
DMM1
DMM2
NI PXI-4070 FlexDMM
System Speed:
Single-Point Measurement
10 VDC Range
Effective Readings per
Second
6
5
4
3
2
1
0
4½
5½
Digits of Resolution
DMM1
DMM2
NI PXI-4070 FlexDMM
6½
Single-Point Measurem ent
4-Wire Resistance, 10 kΩ Range
Effective Readings per
Second
6
5
4
3
2
1
0
4½
5½
6½
Digits of Resolution
DMM1
DMM2
NI PXI-4070 FlexDMM
Effective Readings per Second
Single-Point Measurement
5 VAC Range
6
5
4
3
2
1
0
Slow Filter (10 Hz)
Medium Filter (60 Hz)
Fast Filter (1kHz)
AC Filter
DMM1
DMM2
NI PXI-4070 FlexDMM
Summary
As shown in the graphs above, the FlexDMM is at least 2.5 times faster than the
traditional GPIB-controlled DMMs. They also demonstrate that for a single-point
measurement, DMM configuration time plays a big factor in the overall time. A
DMM's requirements are different from most measurement devices because they
transfer very small amounts of data requiring fast configuration and efficient
communication between the DMM and it controller. Good performance relies on
fast configuration and efficient communication between the DMM and its
controller.
Much of the FlexDMM configuration time efficiency can be attributed to its
register-based configuration setup versus the traditionally slow GPIB messagebased setup. The FlexDMM is configured through a single call in the driver,
which directly writes to the measurement setup registers. Traditional DMMs need
multiple message-based calls that then have to be parsed internally, making the
processor do extra work and slowing setup time. The small latency of the
FlexDMM is primarily due to the streamlined NI-DMM driver software and the 1.2
GHz embedded PXI controller.
Multipoint Measurements
This automated test measures both measurement speed and system speed. The
measurement-speed test measures how long it takes each DMM to initialize,
configure, and acquire 100 points. The system-speed test measures how long it
takes each DMM to initialize configure, scan the entire scan list, taking one
measurement on each channel of the switch, and then transferring the data.
Each channel scanned was a DC voltage in the 10 V range, so no function or
range changes took place. The total number of measurements returned from the
DMM corresponds to the number of channels that were scanned through on the
switch (channel count). A resolution of 4 ½ digits was used and two-way
handshaking was the method of triggering used.
RESULTS
Measurement Speed:
Readings per Second
10 VDC Range, No Switch
(Log scale)
Readings per second
10000
1000
100
10
1
4½
5½
Digits of Resolution
DMM1
DMM2
NI PXI-4070 FlexDMM
6½
System Speed:
Effective Readings per Second
Two-Way Handshaking Multi-Point Measurement
10 VDC Range
500
450
400
350
300
250
200
150
100
50
0
10
100
Full Buffer Length
# of Readings
DMM1
DMM2
NI PXI-4070 FlexDMM
Summary
The DMM1 and DMM2 transfer speeds are primarily dependent on their
communication bus speed and internal data buffer sizes. Hence, to achieve
maximum throughput, the channel count size should equal the DMM buffer
length. For example, if DMM1 has a buffer size of 512 points, then for the full
buffer length scan, one measurement point would be acquired from 512
channels, creating a full buffer and maximizing throughput. The DMMs were
tested for their ability to transfer partial buffers and a full buffer.
DMM1 and DMM2 were connected to the PC via GPIB, which has a maximum
throughput of 1 Mbytes/s. The PXI-4070 FlexDMM uses the 132 Mbytes/s PCI
bus, so it can transfer data more than 100 times faster than GPIB. The ability of
the FlexDMM to transfer at these high rates decreases its dependency on its
internal buffer. Another notable point is that the FlexDMM uses only a fraction of
the PXI bus throughput, which is important when running multiple devices on the
same bus. It is important that that one device not use the entire bandwidth, and
therefore prevent other devices from communicating on the bus. The FlexDMM
also benefits from its extremely fast A/D converter, which can take more frequent
measurements at the rate of 1.8 MS/s.
Multifunction Speed
The ability of a DMM to take multiple measurements and efficiently change
functions and ranges is the key to maximizing test system throughput. For this
automated test, the total measurement time involves the DMM taking
measurements in the following three functions – VDC, VAC, and 4-wire
resistance. For each particular function, multipoint measurements were taken in
three different fixed ranges.
Traditional DMMs have three, fixed AC filter settings which depend on the
frequency of the input signal. This test included three particular cases:
• DC only (only VDC and 4-wire resistance were measured)
• AC medium filter (60 Hz input signal)
• AC fast filter (5 kHz input signal)
RESULTS
System Speed:
Total System Measurement for DC Only
12 (VDC), 12 (4-wire Ω)
Effective Readings per
Second
60
50
40
30
20
10
0
4½
5½
6½
Digits of Resolution
DMM1
DMM2
NI PXI-4070 FlexDMM
Total System Measurement for AC Medium Filter (60 Hz)
450 (VDC), 450 (VAC), 450 (4-wire Ω)
Effective Readings per
Second
40
35
30
25
20
15
10
5
0
4½
5½
Digits of Resolution
DMM1
DMM2
NI PXI-4070 FlexDMM
6½
Effective Readings per
Second
Total System Measurement for AC Fast Filter (5 kHz)
450 (VDC), 450 (VAC), 450 (4-wire Ω)
200
150
100
50
0
4½
5½
6½
Digits of Resolution
DMM1
DMM2
NI PXI-4070 FlexDMM
Summary
Traditionally, AC measurements slow down the total measurement time
considerably, so we have performed both a DC only test and a mixed AC and DC
test. The results clearly indicate that the speed limiting function is VAC. Most
traditional DMMs use an analog rms-to-DC converter for AC measurements,
which adds long settling delays. The PXI-4070 FlexDMM uses a digital algorithm
that only requires a few cycles of a waveform to compute rms values, which
dramatically increases AC reading rates. This digital algorithm automatically
rejects the DC component of the signal, making it possible to offer a DC-coupled
VAC mode that completely avoids the slow-settling input coupling capacitor
required with traditional methods.
The digital approach to rms computation offers accuracy benefits as well. The
algorithm is completely insensitive to crest factor, and can deliver exceptionally
quiet and stable readings. Crest factor is the ratio of the peak value of a signal to
its rms value. For a sine wave the crest factor is 1.414, and for a square wave
the crest factor is 1. This specification is important because it indicates the
maximum peak value of an arbitrary waveform that a DMM can handle without
overloading. The crest factor also affects the accuracy of AC measurements. For
example, given a DMM with an AC accuracy of 0.03% (this is always specified
for sine waves), and an additional error of 0.2% for crest factors between 1.414
and 5, the total accuracy for measuring a triangular wave (crest factor = 1.73) is
0.03% + 0.2% = 0.23%.
Furthermore, the FlexDMM guarantees AC accuracy to 1% of full-scale, rather
than the 10% of full-scale offered by traditional DMMs. To further improve system
speed the FlexDMM uses solid-state relays instead of electromechanical relays,
thereby improving speed and accuracy of function and range changes.
Conclusion
These benchmarks have shown that National Instruments has implemented a
cost-effective 6 1/2 digit DMM that uses PC technology to achieve extremely fast
measurement as well as system speeds in a single-slot 3U PXI module. The NI
PXI-4070 FlexDMM can be used in any PXI system; the user can implement
numerous other measurement devices and switches in one compact chassis. PXI
not only provides fast and controlled triggering across the PXI chassis backplane,
but also it affords users the flexibility of upgrading to a faster measurement
system by using the latest PC processors. National Instruments LabVIEW can
also contribute to performing faster, more powerful postprocessing and data
presentation of the acquired data when it is used in conjunction with the
measurement system.