Test Ideas: Protect USB measurement circuits

USB data-acquisition modules are great for data acquisition, but they often need additional protection circuits when used in production test.

By Lawrence Raymond, Intrinsic Quality, Elk Grove Village, IL -- Test & Measurement World, 10/1/2010 12:00:00 AM

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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.

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 V_CC, or shorted through some low impedance to a higher voltage. Any of these conditions can damage the USB module's input pin.

Figure 1. Simple circuits like this digital driver circuit can have problems that can damage your instrument's DIO ports.

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).

Figure 2. A TVS network, a Zener diode, and a resistor provide protection from excessive voltage and current.

If the DUT has no power sources above the DIO's V_CC voltage, then a simple buffer resistor is all you need to protect the DIO module. The only risk is driving logic into a short at an opposite GND or V_CC state. You should calculate the resistor's value so it lets the circuit function properly but limits current to a safe value when shorts occur.

When the DUT uses higher voltages than the USB module, you need more protection. A fairly common approach uses a Zener diode on the DIO pin and a buffer resistor in series with the DUT. You need to provide current limiting and overvoltage protection. If you use the 96-channel DIO96H module from Measurement Computing, for example, you might think that adding 96 diodes and resistors requires too much effort. Without these parts, though, the DIO ports could fail. For added overvoltage protection, you can add a TVS (transient voltage suppression) network. The DIO96H has pads for mounting the TVS network directly on the module's circuit board.

USB data-acquisition modules also have multiple analog inputs. When you integrate USB modules into test systems, make no assumptions regarding the integrity of the measured signal.

Figure 3. This circuit protects an ADC from high-voltage damage. A voltage divider drops a 24-V signal to 8 V.

The ADC front end of a data-acquisition module can typically handle only ±10 V, so you must consider possible damage from higher voltages. Voltage dividers work, but they reduce the effective resolution of the instrument. You can settle on a compromise: Use a voltage divider in conjunction with back-to-back 9.1-V Zener diodes (Figure 3). The voltage divider scales high-voltage signals into an acceptable range for the ADC. The feeding resistor R_Z protects the Zener diode from excessive current.

USB data-acquisition modules provide analog outputs and a wide variety of other functions. Each of these outputs requires similar protection when used in production test.