Good Switching Maintains Good Signals
by Tom Lecklider, Senior Technical Editor
By definition, an ATE system involves switching. Most often, a minimum set of instruments is provided to support the required types of measurements. Switching is needed to change the connections to the instruments so that more than one voltage or current can be measured. If you only are making one measurement, there’s hardly a need for automation although electronically recording test results does help eliminate human error.
In addition to the numerous multiplexer and matrix switch configurations that are available, a switch’s characteristics must complement the signals it carries. When you consider the various kinds of signals that there are, from high power to dry circuit and DC to RF/microwave, it’s obvious why manufacturers list so many different types of switches.
To date, electromechanical relays provide the best switching performance for almost all types of signals. As an alternative, FET-based switches are available, and for medium-level voltages, currents, and frequencies, they can be a good solution. The main advantages of solid-state switching are an unlimited lifetime and a fast switching speed. The downsides include higher on-resistance, limited current and voltage levels, and a need to protect the FETs from damage.
Relays range from robust armature-style mechanisms suitable for low-frequency, high-current switching to tiny reed-based devices. Reed relays provide very dense switch assemblies for relatively low voltages and currents at frequencies up to tens of megahertz. RF/microwave switches are designed to maintain a constant impedance transmission-line structure. High-voltage relays are physically large enough to ensure sufficient distance between open contacts and use appropriate insulation in their construction.
ATE Switching Examples
Semiconductor Materials Characterization
In this application, described by EADS North America Test and Services’ Senior Product Marketing Manager Charles Greenberg, multiple Racal Instruments Model 1830 Source/Measure Switch systems are involved. The object is to measure inductance, capacitance, resistance, and leakage between thousands of pairs of I/O pins on a test substrate in a high-I/O count package.
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Figure 1. Semiconductor Materials Characterization Test System Diagram |
Mr. Greenberg explained, “The instruments shown in Figure 1 are installed in a rack and cabled to a PCB which can accept devices under test (DUTs) provided by a robotics system for automation purposes. Because capacitance and inductance are being measured, it is important that all cabling remain stationary to avoid frequent recalibration of I/O paths: Cables are tightly tied to a rigid conduit channel.
“The 1830’s internal signal raceway is utilized for multiplexer expansion and signal routing,” he continued. “Resistance and leakage are measured using an external SMU for good clamping to protect the device while an internal 4102 Versa-Meter is used for all capacitance and inductance measurements.
“A cost-effective semiconductor characterization test software framework that easily adapts to different unit under test (UUT) configurations was developed based on the company’s ActivATE® Test Platform. When the test program executes, it makes calls to the high-level .NET drivers that read an Excel configuration file to determine the optimal switch paths required for the test. The test program then downloads a scan list of paths and measurements to the 1830 Source/Measure Switch. Using a scan list instead of discrete close-measure-open sequences results in a very fast and efficient test algorithm,” Mr. Greenberg concluded.
An important part of this test system is the analog bus—the signal raceway Mr. Greenberg refers to—that supports expansion and internal signal routing. Signal fidelity is maximized by eliminating or reducing the need for external cabling and connectors in many applications.
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Switching Platform
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