The system/benchtop DMM is a reliable workhorse in the engineer’s stable of instruments. It offers most of the commonly used test and measurement capabilities, including voltage, current, resistance, capacitance and frequency.
But companies are not resting on the laurels of past product introductions. They continue to offer new and more powerful capabilities to meet current testing needs. For example, many companies now offer an RS-232 interface, 0.0025% basic VDC accuracy and 1,000 readings/s as standard, plus the breakthrough functions that separate them from their competitors.
Recent introductions by Hewlett-Packard broke new ground by providing a nanovolt measurement capability with 8-nV peak-to-peak noise and 100-pV sensitivity at 7½ digits of resolution. The nanovolt capability is combined with a 100-nW function, a standard platinum resistive thermometer and a two-channel VDC input scanning capability on the HP 34420A meter.
Power-line cycle integration time is an important facet on all DMMs from Keithley Instruments. The company’s Model 2010 offers 7½-digit accuracy and noise performance at five power-line cycle integration times. This provides four readings/s with a noise floor of 100-nV rms in the 100-mV range. The noise floor on the 10-V range is 650 nV. Achieving these noise values requires integration times of 20 to 100 power-line cycles for many other DMMs, said Tom Hayden, Operational Marketing Manager at Keithley Instruments. The longer integrating times, however, may make an instrument too slow for production applications.
Engineers also look for the capability to measure critical contact resistance in relays, switches and connectors without breaking down the oxidation common in low-level devices. Low-voltage functions on some DMMs permit contact resistance measurements without breaking down the contact oxidation. For example, an offset-compensated ohms function eliminates the effect of thermal electromotive force in low-resistive measurements, and a dry-circuit ohms capability limits the applied open-circuit voltage to 20 mV for a resistance measurement.
PC-based and VXIbus DMM cards continue to improve and rival their benchtop cousins. The PC-based instrument is available with four-wire ohm measurements, frequency, period, external triggering, level triggering and true rms measurement capabilities, said Tee Sheffer, President of Signametrics. Four-wire ohm measurements are especially important when you use 5½-digit or better test instruments because the IR drop in the test leads causes inaccuracies that become apparent at that level.
Customers are requesting VDC accuracy of 100 ppm, 1 m V and 1-mW resolution; £ 80 dB noise rejection, and 5½ digits of resolution, continued Mr. Sheffer. Applications demanding these specifications include resistance measurements of disk drive heads, noise rejection in manufacturing test and voltage reference for ATE systems, he said.
VXI-based DMMs now supply accuracy of 34 ppm over a year at 10 VDC, said Newell White, Test Instrument Product Manager at Wavetek. Some units, such as the company’s Model 1361, offer VAC rms and VDC measurements to 1,000 V. These attributes make VXI-based DMMs a good functional match for the six-digit rack-and-stack systems, added Mr. White.
DMM customers also ask for a voltage-comparison capability. Traditionally, this task required that you make the first connection, take the measurement, then do the same for the second point before comparing the two results. A built-in two-channel programmable VDC scanner, like the one offered by Hewlett-Packard, allows two DC voltages to be automatically scanned and measured with the ratio or difference displayed.
Software or firmware is another capability to consider for your system/benchtop DMM. One of the most requested software features connects the DMM to a PC and allows you to set up and initiate measurements as well as to retrieve data and analyze it, said Mr. Scott. These software packages get the DMM measurement data into your PC and allow you to create graphs, move test results into various Windows-based programs, and perform basic statistical analysis without writing a single line of code.
Trends
Many years ago, interfaces such as RS-232 and GPIB were optional on DMMs. Today, interfaces are a necessity, even if the DMM is used for manual applications because the calibration may be performed automatically. An interface also allows you to use the same DMM in both automated and manual systems.
Providing integrated test systems is another trend in the industry, said Mr. Sheffer. PC-based instruments help eliminate additional equipment along with their extra power cords, cables and interface cards. Now they meet the performance requirements of large companies with the need for high throughput.
Another type of integrated system is supported by VXI-based test instruments. The VXI-based DMM market is driven by manufacturing companies seeking improved functional test systems with enhanced throughput. VXI often is selected because of data transfer rates higher than rack-and-stack systems and better triggering and synchronization between modules.
System/Benchtop DMMs
Meter Provides Frequency
And Temperature Functions
The 5½-digit DP100 DMM measures voltage, current, resistance, frequency and temperature. It offers an accuracy of ± 0.003% for DC voltage, 0.25% for AC voltage and 0.007% for resistance. The instrument checks frequencies to 25 MHz and responds to signal transitions as low as 200 mV. It measures temperatures from -200° C to +250° C via RTD sensors. The meter is calibrated via front-panel keys and a reference standard. An RS-232 interface allows remote operation with the optional DP-Link™ software package. $595. Analogic, (508) 977-3000.
Meter Has ½”-High Display
And Measures to 20 A
The Model 2831A is an AC-operated 3½-digit bench meter with a 0.1% voltage accuracy, a VAC response to 40 kHz and a ½”-high LED. It measures current to 20 A, voltage to 1,200 VDC or 1,000 VAC and resistance to 20 MW . Resolution is 0.1 mV, 0.1 A and 0.1 W . A continuity check emits a tone for resistances <10 W . All functions are overload protected with high-energy fusing for current ranges. $299. B+K Precision, (312) 889-1448.
Meter Has
100-pV Sensitivity
The HP 34420A NanoVolt/Micro-Ohm Meter has the capability to perform nanovolt and microhm measurements with 100-pV sensitivity. The instrument has full scale DC ranges from 1 mV to 100 V, resistances from 1 W to 1 MW . The unit performs 250 readings/s at 4½ digits to 0.15 readings/s at 7½ digits. It includes an integrated current source and a scanner for two-channel DC voltage-measurement applications. Two-wire and four-wire resistance measurements are offered. $3,195. Hewlett-Packard, (800) 452-4844.
Unit Combines DMM Functions
With Source Capability
The Model 2400 Digital SourceMeter™ combines voltage and current sources with a DMM and measurement firmware. It has a DC voltage and current-source accuracy of ± 0.012% and 5½-digit resolution. The unit’s source-delay-measure speed capability allows 1,000 nonbuffered readings/s at 4½ digits or 500 readings/s at 5½ digits. Measurement settling time is 500 m s. Source and measure voltage capabilities extend from ± 1 m V to ± 200 VDC, and current ranges from ± 1pA to ± 1 A. The instrument stores 100 measurement configurations containing the source setting, sense measurement and pass/fail criteria. A guarded four-terminal measurement capability is provided. $3,495. Keithley Instruments, (800) 552-1115.
PC-Based DMM Offers
5½-Digit Resolution
The SM-2020 is a PC-based DMM offering 5½-digit resolution and a 22-bit analog-to-digital conversion capability. The instrument operates via a graphical user interface and makes low-level measurements without the need for signal conditioning. It offers four DC voltage ranges from 300 mV to 300 V with 1-m V resolution in the lowest range. AC measurements incorporate true rms conversions with a bandwidth from 10 Hz to 100 kHz. The unit supplies a four-wire sensing capability and measures as low as 1 mW . A DC-to-DC converter isolates critical analog circuits from the host computer. $995. Keithley MetraByte, (800) 348-0033.
Multifunction Meter Offers
7½-Digit Resolution
The 8017 Multifunction Meter measures AC/DC voltage and current, two-wire and four-wire resistance, frequency, period and temperature. Temperature is measured by platinum-tipped probes or thermal elements. The unit has 7½-digit resolution with a stability of 4 ppm over 24 h for DC voltage measurements. Sample times range from 1 ms to 100 s. It stores up to 40 measurement series with 15,000 values each. $3,600. Prema Precision Electronics, (800) 441-0305.
Half-Size DMM Card Provides
4½-Digit Resolution
The SM-2010CT is a PC-based DMM occupying a ½-size ISA slot and offering 4½-digits of resolution. The instrument has DC voltage ranges from 250 mV to 250 V with a resolution ranging from 10 m V on the lowest setting to 10 mV on the highest. DC accuracy is 0.03%. Its resistance ranges extend from 250 W to 25 MW with 0.05% accuracy. The AC and DC current extends from 25 mA to 2 A with 0.3% accuracy and 3-A overload protection. AC voltage and current are true rms measurements. The AC voltage range extends from 250 mV to 250 V with a frequency response range from 20 Hz to 100 kHz and an accuracy from 0.5% to 8%. Readings range from 1 reading/s to 60 readings/s. $795. Signametrics, (206) 524-4074.
VXIbus DMM Measures
AC and DC Voltages to 1,000 V
The Model 1361 is a 6½-digit multifunction VXIbus card DMM that accepts inputs to 1,000 VDC or AC rms and measures resistances to 20 MW . The instrument provides isolation between the measurement circuits and the VXIbus backplane. The front panel can float as much as 650 V above or below ground potential. The common-mode rejection ratio of the unit is 146 dB. Switchable filters provide a normal-mode AC rejection of 20 dB. It makes 1,000 readings-per-second at 4½ digits and 5 readings-per-second at 6½ digits. $4,995. Wavetek, (800) 223-9885.
Series Supports Production,
Maintenance and Design
The 4½-digit 740 Series Benchtop DMMs offers three versions for different applications. The Model 740 01 has a comparator function for production line testing and has a two-step measurement period of 2´ /s to 4´ /s or 5´ /s to 10´ /s. Measurement functions include AC/DC voltage, resistance, averaging, measuring period, data hold and automatic hold. The 740 02 features a nickel-hydrogen battery unit for maintenance operations and a peak-hold function. The 740 03 provides high-performance multiple functions for design use. The 740 02 and 03 units offer measurement functions including AC/DC voltage and current, resistance, continuity, frequency, temperature, range hold, averaging, peak-value hold, data hold and automatic hold. 740 01: $799; 740 02: $899; 740 03: $799. Yokogawa, (800) 258-2552.
Copyright 1996 Nelson Publishing Inc..
August 1996