Originally published December 1996
Unlike many types of electronic products, medical equipment such as defibrillators and pacemakers must meet unusually high standards for performance; their failure can cause injury or death. Pacemakers typically remain dormant for extended periods, waiting for a signal, so failure is often not detected until too late for a routine response. Defibrillators must deliver high-voltage pulses on demand, often in harsh evironments. Hearing aids, while not crucial for maintaining life, must also function reliably, and their extremely dense construction and diminutive size require that component tolerances be tighter than those of many other products.
A key factor in the safety of these devices is impedance, the opposition to ac current flow offered by a device or circuit. A complex value made up of resistance, inductance, and capacitance, impedance can help determine whether a device or circuit will function to specifications. Unfortunately, its role in device reliability is often overlooked; in fact, industry standards do not even specify necessary impedance values. Yet impedance should not be ignored as a safety factor.
With their special safety requirements, medical electronics can benefit substantially from impedance measurement.
Many medical equipment manufacturers routinely perform impedance tests on their components at one or more stages in production, as early as incoming inspection and as late as final testing (see Table I). However, it is likely that just as many manufacturers rarely if ever perform these tests.
One reason for this is undoubtedly that many design engineers think impedance measurements are difficult to conduct and require a toolroom full of instruments and a tangle of cables, banana plugs, and baby Neill Concelman connectors. While that was certainly true a decade ago, today, thanks largely to advances in digital signal processing and semiconductor technology, there are instruments available that make impedance measurement clean and simple. They can display the resistance, inductance, and capacitance parameters of an impedance measurement as well as calculate and display many other related parameters, such as impedance, yielding admittance, reactance, conductance, susceptance, and dissipation.
In the past, the instruments used to perform impedance tests were either meter types, which measured voltage or current and left it to the operator to calibrate the impedance quantities, or bridge (null) types, which required an adjustment and indicated impedance on a scale or dial associated with a variable component of some kind. Meters were faster and easier to use, but bridges were more accurate.
Today's instruments have the best characteristics of both types of instruments, with few of their eccentricities. Digital in design, these new machines provide the typical array of features of this type of equipment, such as Institute of Electrical and Electronics Engineers (IEEE) bus control, nonvolatile memory for instrument setups, broad configurability, ease of use, and the ability to be upgraded in place.
Because impedance measurement requires rapid and accurate ratio calculations, microcomputers are well suited for this testing and are the basic component of all modern impedance equipment. While these new instruments perform the functions of a bridge, they lack a null function, instead successively measuring the voltage across the unknown resistor and the voltage across a standard resistor carrying the same current, and then dividing the results.
Four helpful features available in today's instruments include swept parameter measurement capability, menu programming, test data collection, and setup storage. Swept parameter measurement provides values over a user-specified frequency range and delivers the information in graphical or tabular format via the display or printer. Menu programming allows operators to access test conditions with the touch of a button. Test data collection allows test results to be stored automatically on an internal diskette drive or transferred to a PC for further analysis. Setup storage allows operators to store commonly used test routines.
TIPS ON HANDLING AND MEASUREMENT
Thanks to these new instruments, making impedance measurements today is relatively simple. However, care must still be exercised to ensure reliable results.
Especially important for surface-mount manufacturing is the need for careful handling of small parts. The size of these devices often makes it difficult to obtain good electrical contact. One of the most useful tools is the Kelvin clip cable, which allows four-terminal connections to be made to passive components. It is especially useful for testing low impedances encountered with components such as electrolytic capacitors and inductors.
A set of chip component tweezers that plug directly into the instrument is also helpful. The tweezers allow the devices to be picked up, measured, and placed in the proper bin in one operation.
The same results can be obtained with a component test fixture. Chip component fixtures ensure that tiny devices are nestled securely so measurements can be made easily and quickly. The same level of electrical contact obtained with Kelvin clip cables can be obtained with a test fixture.
Remote high-voltage test fixtures are necessary when high voltages are required to make the measurement. A safety interlock allows the person making the measurement to place a component in a fixture, make the measurement, and remove the component without risk.
CHOOSING THE NUMBER OF CONNECTIONS
Impedance measurements can be made using 2, 3, 4, or 5 terminals. Two-terminal measurements are easiest and typically are used in the impedance range of 100 to 10 k. Three-terminal measurements (made with two terminals and a guard) are useful for high-impedance measurements when the effects of stray capacitance can introduce errors as well as when guarded, in-circuit measurements are necessary.
Four-terminal measurements (with two current and two potential terminals) are required to obtain accurate impedance measurements below 100. The method eliminates series impedances and contact-resistance errors. Five-terminal measurements are made when results over a wide impedance range are desired.
When low impedances are measured, a two-terminal connection introduces errors caused by the addition of the series impedance of the connecting leads. As a result, instruments now use the four-terminal method to perform low-impedance measurements.
The terminals carrying the current are called the current terminals and those used to make the voltage measurement are called the potential terminals. The proper method for a given application is determined by the measured impedance value and the accuracy required.
OTHER TEST CONSIDERATIONS
The basic measurement parameters such as voltage, frequency, and equivalent circuit are determined by national and international standards and manufacturer specifications. Generally, low impedances such as large-value capacitors and low-value resistors and inductors are measured using a lower frequency, such as 100 Hz or 1 kHz, in the series configuration. For small inductance, higher measurement frequencies are required.
Because many instruments allow the user to select measurement speed, it is tempting to attempt the measurement in the shortest possible time. However, the higher the measurement speed, the lower the accuracy. Instruments that offer autoranging will automatically find the proper range for a given measurement, but locating the proper range can add time to the measurement.
Medical devices such as defibrillators and pacemakers must perform unfailingly, often without periodic testing. Thus, manufacturers must perform an exhaustive battery of tests as they are developing and producing these types of devices. Incorporating impedance measurements in these test routines is an important part of ensuring a high level of performance, and thanks to advances in measurement equipment, the impedance tests can now be performed easily.
Jim Richards is the marketing engineer at QuadTech, Inc. (Marlborough, MA).