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Test Ideas: Protect USB measurement circuits

USB data-acquisition modules offer good value and ease of use, which makes them an attractive choice for manufacturing test. But before you use the modules in a manufacturing test system, you need to take steps to protect them. During manufacturing test of circuit boards or subassemblies, a defect in an assembly may result in a condition that damages a data-acquisition module.

Protect USB measurement circuits

The typical USB DIO (digital I/O) module uses a set of 8-bit bidirectional tristate ports. Figure 1 shows a typical circuit that we test with one of those ports. We use one digital output pin from the USB module to drive the circuit and one digital input pin to read the circuit’s response. If the circuit works properly, we expect to see a clean digital signal from the circuit. For a circuit coming off the manufacturing line, however, the digital input pin of the DUT (device under test) could be shorted to ground, shorted to VCC, or shorted through some low impedance to a higher voltage. Any of these conditions can damage the USB module’s input pin.

If there’s a short across the 2-k resistor from the collector to the +15-V supply, that voltage will appear at the USB module’s input pin, which could damage it. To avoid such damage, we add protection circuits between the DUT and the USB module (Figure 2).

Protect USB measurement circuits

If the DUThas no power sources above the DIO’s VCC voltage, then a simplebuffer resistor is all you need to protect the DIO module. The only risk isdriving logic into a short at an opposite GND or VCC state. Youshould calculate the resistor’s value so it lets the circuit function properlybut limits current to a safe value when shorts occur.

When the DUT uses higher voltages than the USB module, you need moreprotection. A fairly common approach uses a Zener diode on the DIO pin and abuffer resistor in series with the DUT. You need to provide current limitingand overvoltage protection. If you use the 96-channel DIO96H module fromMeasurement Computing, for example, you might think that adding 96 diodes andresistors requires too much effort. Without these parts, though, the DIO portscould fail. For added overvoltage protection, you can add a TVS (transientvoltage suppression) network. The DIO96H has pads for mounting the TVS networkdirectly on the module’s circuit board.

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