Avoid switching mistakes
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 9/1/2007 2:00:00 AM
Switching systems appear in many automated test systems used for both prototype evaluations and production tests. Even the most carefully planned switching designs can have issues that degrade performance.
Dale Cigoy, senior applications engineer at Keithley Instruments, helps engineers solve switching problems every day. He’s written a paper called “Designing and Building the Perfect Switching Test System.”
| Download a copy of Cigoy's paper, “Designing and Building the Perfect Switching Test System.” |
In his paper, Cigoy covers common issues such as contact resistance, settling time, channel-to-channel isolation (figure), and switch aging. He also discusses solid-state switches versus electromechanical relays, pointing out that solid-state switches have far greater life than relays, but by using them you pay a price in contact resistance.
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A switch card’s channel-to-channel isolation represents how well it guards against signal crosstalk. |
“Trying to measure a few milliohms with upwards of 10 ohms of resistance in the circuit from the 'on’ resistance would effectively bury the low-resistance measurement,” warns Cigoy. He then points out that you can negate high resistance by using a “golden” channel and subtracting the resistance or by using four wires to separate system resistance from the measuring circuit.
Cigoy also discusses switch topology and compares multiplexed and matrix configurations. He covers full, partial, and blocking matrices and says, “The advantage of matrix configurations includes the absence of unterminated stubs, access to all channels, and similar path characteristics. Disadvantages include the need for extensive cabling and the use of many coaxial relays.”
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