KEITHLEY.NOV–Keithley–wy
DMM performance continues on the upswing
Significant improvements in performance and price are made possible by
extensive use of ASICs and off-the-shelf processors
BY MARK CEJER Keithley Instruments, Inc. Cleveland, OH
Over the past decade, users have continually migrated from
lower-performance to higher-performance digital multimeters. This shift is
the result of the growing availability of higher-performance DMMs at
increasingly lower prices and users' demands for higher-performance DMMs
to test their own higher-performance products. Thus, DMM capabilities that
only a few years ago were thought of as leading edge are now considered
standard. Perhaps the most striking change in DMM architecture over the
past few years is the growing use of multiple processors (see Fig. 1). In
the past, DMMs typically employed just one (a few used two)
microprocessors to control the instrument's five major functions: A/D
conversion, display, triggering, front-panel control, and GPIB interface
(see Fig. 2). For the most part, this level of design was considered
perfectly adequate and cost-effective just a few years ago. And it still
is for applications where 8-bit A/D resolution, modest measurement speed,
and GPIB throughput are acceptable. However, user demands increased for
DMMs with more functions. These functions include tighter A/D control,
higher accuracy, more precise triggering, easier-to-use-instruments, and
GPIB communication that doesn't slow down instrument speed. It became
obvious that this level of capability could best be provided by DMMs with
multiple processors. Today's high-performance DMMs employ multiple
processors to handle different functions or groups of functions.
Keithley's 7 1/2-digit Model 2001 DMM is a good example of this kind of
instrument. The 2001 incorporates five different processors, each designed
to handle separate tasks (see Fig. 3). A Motorola 68000 16-bit
microprocessor performs high-speed command processing and calculations. A
second chip, the 68302, combines a reduced instruction-set computer with a
68000 communications processor. The front panel has its own microprocessor
to respond instantaneously to keystrokes and to ensure that the time
display doesn't affect measurements performed elsewhere in the instrument.
Another dedicated measurement processor in an application-specific IC
provides tight control over A/D conversion. Finally, an ASIC trigger
processor makes possible high-speed triggering without the timing
uncertainties found in earlier architectures. These processors make it
possible to handle individual tasks without becoming bogged down by other
demands on the circuit, allowing for better measurement performance and
higher throughput. Only a few years ago, adopting a multiple-processor
architecture would have been considered cost-prohibitive. However, the
rapidly declining cost of ASICs and the introduction of relatively
inexpensive multifunction integrated processors have made these
architectures viable.
Features and capabilities expand The radical changes in DMM architecture
have made it possible for instrument manufacturers to incorporate a wide
variety of new or improved performance features into the instruments. One
of the most beneficial of these design innovations has been higher
throughput. For example, Keithley's Model 2001 DMM and Hewlett-Packard's
Model HP 34401 6 1/2-digit DMM both provide 2,000 readings/s (at 4 1/2
digits). Both the Keithley and Hewlett-Packard DMM can thus be employed in
high-speed production test applications or where the user must capture
fast-moving signals. However, ensuring high system throughput depends on
much more than just the instrument's reading rate. Manufacturers are also
striving to minimize trigger latency and uncertainty, reduce the time
required to make range and function changes, speed up autoranging
functions, and shorten overload-recovery times. DMM display technology
has also evolved, providing substantially more usable information than the
displays of the past. Over the past few years, single-line LED displays
have given way to dual-line, character-based vacuum-fluorescent displays
(VFDs). DMMs ranging from the low-cost Fluke 45 to the high-end Datron
1281 have incorporated dual-line VFDs to provide the user with additional
information, such as ac volts and frequency or dc volts and ac ripple for
power supply testing. And many of today's front-panel displays incorporate
multiline VFDs and can be configured to provide information in a variety
of formats, such as showing the results of up to three different types of
measurements on the same signal simultaneously or providing a bar graph
for nulling. Changes in the way users interact with DMMs have had an
impact on instrument design. As a rule, modern users try to avoid
referring to the operator's manual whenever possible, unless absolutely
necessary. To accommodate this tendency, manufacturers have begun to add
context-sensitive Info functions to their instruments to reduce the need
to consult the manual. The uncertain user simply presses a button for
instructions on how to proceed with a measurement. Along with the greatly
increased performance of DMMs, there is a trend toward compactness. More
functionality is being housed in less space. DMM capabilities that once
required a full-rack case can now fit into a half-rack box. Such
compactness is largely due to the high-circuitry density of ASICs and
other microprocessors, as well as the use of surface-mount components.
These economics have provided breakthroughs in price/performance ratios
for today's users. For example, the HP 34401 (see Fig. 4) provides 6 1/2
digits of resolution and very fast reading rates, yet it costs less than
$1,000.
New functions are appearing One of the most significant design trends in
today's high-performance DMMs is the incorporation of several functions
never before available.
Ac crest factor measurement. This function indicates if a measured
signal's crest factor (the ratio of peak of rms volts) is affecting the
instrument's specified accuracy. The ac-volts accuracy of most DMMs is
specified for a sine wave input. However, if the input signal is
non-sinusoidal, the user must account for additional crest factor errors.
The ability to measure the ac crest factor allows users to identify these
errors without the need for an oscilloscope to verify the shape of the
input signal.
In-circuit measurement. Unlike voltage and resistance measurements, which
can be made in parallel, measuring current has traditionally required
breaking the circuit to connect the meter in series with the signal path.
In-circuit current measurement capabilities, available in the Model 2001
DMM, make it possible to measure the current flowing through a wire or a
circuit board trace without breaking the circuit to insert the meter.
Peak spike measurement. This function essentially expands voltage spikes
to provide a measurement-compatible signal to the DMM's A/D converter.
This makes it possible to measure high-level transients with extremely
short pulse widths. In many applications, this capability can make an
oscilloscope or high-speed digitizer unnecessary, allowing simpler
measurement setups and significant cost savings.
Direct temperature measurement. Temperature is one of the most commonly
measured physical variables, and is often measured with electrical
parameters. Many DMMs now support directly measuring temperature with high
resolution by means of the user's choice of RTDs or thermocouples. DMMs
now on the manufacturers' drawing boards will doubtlessly boast further
improvements in performance–for example, high-resolution at lower cost,
greater accuracy, faster reading rates, and more function displays. Two
possible near-future developments, though, would significantly improve DMM
performance and help reduce user costs. The first is built-in capabilities
that will allow system DMMs to control the test and measurement process
independently, without the need for an external PC. This will save
considerable setup time and costs. For example, the addition of real-time
clocks will allow users to trigger measurement events or time-stamp stored
data. Also, new mathematical functions will make it easier to analyze
acquired data without the need for additional equipment. Another
important future goal is to reduce the frequency of calibration needed to
maintain the user's required level of accuracy. Obviously, by lengthening
the time interval required between calibrations, downtime and support
costs can be reduced significantly.
CAPTIONS:
Fig. 1. The Model 2001 7 1/2-digit DMM from Keithley Instruments employs
five separate processors to control each of its five major functions.
Fig. 2. Until a few years ago, almost all DMMs were based on a
single-processor architecture.
Fig. 3. The Model 2001 DMM incorporates five processors, one to control
each major DMM function.
Fig. 4. Hewlett-Packard's HP 34401 DMM, which provides 6 1/2 digits of
resolution, costs less than $1,000.
Advertisement
