As an engineer, you need to know what instruments provide fast measurement speeds to keep up with the demands of production test. You need to know what instruments will improve your throughput and which combinations of functions will get your products tested faster.
Precise measurements and versatile capabilities are essential to the success of any test line. Production test measurements typically include the basic AC and DC tests of voltage and current with quick changes to check resistance using two- or four-wire measurement techniques.
Many system and benchtop DMMs also allow you to verify frequency and period or to test diodes. But add to this the test requirements for measurement accuracies from 4 1/2 to 8 1/2 digits and quick setup times and measurement speeds, then choosing the best DMM becomes a knotty issue.
Elements of Measurement Speed
Instrument measurement speed is important in many test situations. When specified, measurement speed is usually stated as a specific number of readings per second. Other factors, such as integration period and the amount of filtering, also affect measurement speed.
If you want to improve your measurement speed, consider the time it takes the instrument to connect, settle, autorange, integrate and process, as well as the time to recover from overloads and the interval of the trigger latency.
The connect time is the duration required to contact the DUT. The connect time includes the actuation time of the switching elements plus the trigger latency of the switching system.
The source and DUT settling times are the delay intervals required for source or DUT time constants to settle to the required resolution. Autorange indicates how long it takes the instrument to identify and switch to the optimum measurement range after the signal is applied.
Integration time is how long the analog-to-digital converter samples the test signal. Many instruments have a selectable integration time allowing optimization of measurement speed. Generally, there is a trade-off among integration time, resolution and the level of noise. For example, as integration time decreases, noise increases and resolution decreases.
The processing time of an instrument includes all the internal overhead calculations starting from the time the analog-to-digital conversion is completed until the reading is displayed. It encompasses the duration needed for the internal analog-to-digital calculation plus the time it takes to perform any additional math operations, such as ratio or resistance.
The overload recovery interval is the time required for an instrument or other part of the test system to recover from an excess condition, including voltage or current.
Trigger latency also affects measurement speed. It defines the time duration from a trigger stimulus to the beginning of the process it initiated. Typical trigger stimuli include external trigger pulses, front-panel keys and IEEE 488 bus commands.
Features to Reduce Setup Time
Most DMMs are used with other instrumentation, such as switching or scanning systems, that allow multiple devices to be rapidly tested. To reduce the time it takes to get the complete test system running, consider how easily the system components are integrated, said Mark Hoersten of Keithley Instruments. Specifically, determine how well the DMM and the switching/scanner system provide physical interconnection and functional compatibility.
For example, if the number of devices to be tested is £ 10, consider a DMM with an optional scanner card, said Mr. Hoersten. Otherwise, the DMM switcher and scanner system needs compatible electronic connectors and should be rack mounted.
Functional compatibility indicates how easily you can program the DMM and switching system to work together with appropriate trigger inputs and outputs, added Mr. Hoersten. The DMM trigger I/O also should support other instruments so all hardware can be easily tied together.
Features to reduce setup time depend on the DMM application, said Barry Scott of Hewlett-Packard. For example, if you need a general-purpose benchtop DMM, look for these features:
Transportability–The instrument must be easy to move if it will be shared by multiple users. Size, weight and mechanical ruggedness are the key features.
Functionality–The useful functions include diode test, continuity check, limit test, built-in math and reading hold. These functions are aimed at customers doing benchtop troubleshooting and device checks.
Front-Panel Interface–The design of the user interface directly affects system setup time. Commonly used functions must be easy to set up and initiate. The front-panel arrangement must provide full access to the DMM’s functionality but not encumber you with extra complexity and button presses when using the most popular features.
Connectivity–Sometimes you need to document or record a series of measurements, such as monitoring the temperature rise of a component over time. Rather than watch the display for hours with a pencil in hand, a connection to a PC-based software package can help you perform the task with a minimum of effort.
Product Reliability–This is an often overlooked but pivotally important aspect for reducing system setup time. It means DMMs must be electrically robust with a protected front end that can handle input overloads without being damaged. They also must withstand the usual knocks, bumps and accidental abuse.
The system DMM is in a different instrument classification than the benchtop DMM. It requires additional and sometimes different features to meet your needs, said Cliff Asbill, Senior Product Specialist for Fluke. Speed is a key issue for a full production/rack-type system. Another factor is the need to communicate with several instruments on the same interface port. Often this is accomplished with instruments containing an IEEE 488 interface.
Programming is another feature you should consider for a system DMM, especially if you pound out your own code, said Mr. Scott. If you create your own code, make sure it is compatible with your existing products. This allows you to leverage existing code or develop new code.
To measure low-level resistance would require an external current source to supply a known current through the DUT and the nanovoltmeter connected across the DUT to sense the voltage. Then the DUT resistance value and measurement error must be calculated.
Low-level resistance also can be measured with a built-in current source with measurement sensitivity to 100 nW , said Mr. Scott. This saves system setup time and the extra expense for an external current source.
The automatic ratio and difference measurement is for precise comparison of two DC voltages. Typically, you have to make two connections and two separate measurements and compare the results.
An alternative method is available with the HP 34420A two-channel programmable DC voltage scanner. It automatically scans and measures two voltages and displays the ratio or difference of the voltages.
Accurate temperature measurements are another feature to consider if you are looking for a low-level measurement DMM. These measurements usually are performed with resistive transducers, such as standard platinum resistive thermometers. Typically, the transducers are 100 W or less at ice-point and require noise measurement resolution of 7 1/2 digits to take advantage of their accuracy.
Although the test capabilities are different for the system and benchtop user, they share measurement speed, setup time and test throughput. They will help you stay in the fast lane of test.
1. Low-Level Measurements, Precision DC Current, Voltage and Resistance Measurements, Keithley Instruments, 1992.
Benchtop Unit Provides
Backlit LCD and Bar Graph
The Model 2835 DMM offers a tilt stand, a neck strap, a 1,800-h battery life, a built-in storage compartment and a bar graph. The 3 3/4 backlit LCD unit has a relative mode, a data hold and diode test, and provides minimum/maximum measurements. Basic DC voltage accuracy is 0.5%. Measurement ranges include capacitance from 0.001 nF to 40 µF, resistance from 0.1 W to 40 MW , frequency from 0.01 Hz to 1 MHz, AC voltage to 1,000 V, DC voltage to 750 V and DC/AC current to 10 A. High-energy fusing is provided on all current ranges. $270. B+K Precision, (312) 889-1448.
Meter Provides 5-Digit
The Fluke 45 Multimeter is a 5-digit, 100,000-count unit with a dual display that allows measurement of two signals from one test connection. Measurements include true rms voltage from 100 mV to 750 V, DC voltage from 300 mV to 1,000 V, resistance from 300 W to 100 MW , current from 10 mA to 10 A and frequency from 1,000 Hz to 1 MHz. The unit also offers a diode/continuity test, a decibel readout with selectable reference impedance and a compare function for in-tolerance tests. $659. Fluke Corp., (800) 443-5853.
Meter Provides 6 1/2-Digit
Resolution and True RMS
Hameg’s HM8112-2 Digital Multimeter is a 6 1/2-digit, 1,999,999-count unit with true rms measurement capabilities. It provides 10 readings/s in all AC ranges and has a frequency range from DC to 100 kHz. Common-mode rejection of 140 dB is ensured by separate guard connections. Resistances are determined by two-wire or four-wire measurements with a resolution of 1 mW . Mathematical routines to calculate and display percent deviation and dB/dBm values are provided with equipment firmware. Integration options are 100 ms, 1 s and 10 s. $1,525. Hameg Instruments, Inc., (800) 247-1241.
The HP 34420A NanoVolt/Micro-Ohm Meter has the capability to perform nanovolt and microhm measurements with 100-pV sensitivity. The instrument measures DC voltages from 1 mV to 100 V; resistances from 1 W to 1 MW . The unit performs 250 readings/s at 4 ½ digits to 0.15 readings/s at 7 ½ 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 Co., (800) 452-4844.
Benchtop Unit Offers
The GDM-8055G Benchtop DMM provides six measurement functions and an IEEE/GPIB interface. It measures DC voltages from 200 mV to 1,000 V with a resolution ranging from 1 µV to 10 mV. True rms AC voltage measurements are provided from 200 mV to 750 V. The DMM measures DC current from 200 µA to 2 A with a resolution from 1 nA to 10 µA. The unit also measures true rms AC current from 200 µA to 2,000 mA and resistance from 200 W to 20 MW . Software calibration is performed from the front panel or remote interface. The unit offers data logging, relative mode and min/max features. $899. Instek Corp., (818) 336-6537.
Low-Noise DMM Provides
13 MeasurementsThe Model 2000 Digital Multimeter is based on high-speed, low-noise A/D converter technology. It has 13 built-in measurement functions including AC/DC voltage, AC/DC current, two- and four-wire ohms, continuity, period, frequency, dB, dBm, temperature and diode test. At 6 ½ digits, the instrument delivers 30 triggered readings/s; at 4 ½ digits, it makes 2,000 readings/s. It also autoranges in <30 ms and changes ranges at 50/s. $995. Keithley Instruments, (800) 552-1115.
Unit Features Measurement
The Model 856 DMM is an autoranging, 4 1/2-digit portable meter offering voltage, current, resistance, continuity, diode measurement functions, frequency and a bar graph display. Additional features include a comparison of measured results and calculated functions for deviation, relative and decibel measurements. AC voltage and current readings are made via a true rms measurement technique. DC voltage ranges from 300 mV to 1,000 V with a resolution from 10 µV to 100 mV. AC voltage measurements are from 3 V to 750 V and frequency measurements from 5 Hz to 300 kHz. $845. Leader Instruments Corp., (516) 231-6900.
Benchtop Unit Offers
The OM7551 and OM7552 DMMs are desktop models that use feedback-pulse width modulation to provide stability, noise immunity and linearity. They provide an RS-232 or GPIB communication interface. Sampling is performed using the standard memory of 1,000 data points or 8,000 data points with the optional memory card. Units read AC/DC voltage and current as well as resistance. OM7551: $895; OM7552: $995. OMEGA Engineering, Inc., (203) 359-1660.
Multifunction Meter Offers
7 1/2-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 a 7 1/2-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, Inc., (800) 441-0305.
Meter Features Frequency
And Programmable Trigger
The SM-2020CT DMM features 5 1/2-digit measurement capability and frequency, hardware and programmable level triggering functions. Frequency and period measurements are made with 5 digits of resolution for AC voltage and current. The external hardware trigger and the programmable level trigger define the threshold to capture low-frequency waveforms. The unit provides 1 to 200 readings/s, two- or four-wire resistance measurements, and 300 mVDC to 300 VDC measurements with 1 µV resolution. $995. Signametrics, (206) 524-4074.
Meter Has GPIB Interface
And 0.03% Accuracy
The DM2510G Programmable DMM provides a GPIB interface, six-function autoranging capabilities and 0.03% accuracy. The 4 1/2-digit meter is programmable from the front panel, offers data hold and has an over-range indicator. DC voltage measurements are from 200 mV to 1,000 V with ³ 80 dB common-mode rejection at 50/60 Hz. AC voltage measurements are from 200 mV to 500 V with a ³ 60-dB common-mode rejection ratio at 50/60 Hz. DC current measurement are from 200 µA to 10 A. AC current measurements are from 200 µA to 10 A. $725. Tektronix, Inc., (503) 627-7111.
Benchtop Unit Offers
Voltage Measurement to 1,200 V
The Model BDM40 is a 4 1/2-digit benchtop DMM featuring AC and DC voltage measurements in five ranges to 1,200 V, six ranges for current to 20 A, six resistance ranges to 20 MW and a diode test. A push button selects a true rms measuring circuit for AC or AC+DC coupling. Basic accuracy of the unit is ±0.03% of reading. $429. Wavetek Corp., (619) 279-2200.
Memory Storage Card
The 7550 and 7560 Series Benchtop DMMs offer multifunction capability and a dual memory mode. The internal memory holds up to 1,000 data points. The 7550 Series features a 5 1/2-digit display with ±0.005% accuracy. The 7560 Series provides a 6 1/2-digit display with ±0.003% accuracy. A feedback pulse-width modulation A/D converter enhances noise immunity, accuracy, stability and linearity. Measurement functions include DC and AC amps and volts, frequency, current and resistance. Start at $995. Yokogawa Corp. of America, (800) 258-2552.
Meter Provides Frequency
And Temperature Functions
The 5 1/2-digit DP100 DMM measures voltage, current, resistance, frequency and temperature. It offers accuracy of ±0.003% for DC voltage, 0.25% for AC voltage, and 0.007% for resistance. The instrument measures frequency 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. $595. Analogic Corp. (508) 977-3000.
Copyright 1995 Nelson Publishing Inc.