Using USB for test and measurement: benefits and potential dangers

Recognised for its ease of use and flexibility, USB is one of the fastest growing buses within the computer and electronics industries today.

All new PCs have a number of USB ports, some with as many as six, for connecting external devices. For test and measurement applications, USB data acquisition modules offer a number of significant advantages. But beware — they also contain some potential dangers that can cause catastrophic results depending on your application.

Benefits of using USB for test and measurement

USB provides users with a simple alternative for developing test and measurement application by offering the following advantages:

• True plug and play: Simply connect your data acquisition module to the USB port of your PC using a standard, low-cost cable. The PC automatically identifies the module when it is plugged in and installs the software necessary to operate it. This connection scheme greatly reduces your start-up time. You don't have to open your PC to add a board, configure DIP switches and IRQ settings, search for the right device driver or reboot your system. Simply connect your sensors to the module and go. Within minutes, you're acquiring data: temperature, pressure, sound — whatever you need.
• Less noise sensitivity from the PC: USB data acquisition modules offer performance benefits for noise-sensitive measurements. Since the USB cable is typically one to five metres long, the I/O circuitry is located further away from the computer's noisy motherboard and power supplies, and closer to the sensors they will be measuring.
• Full- and high-speed transfer rates: Computers configured with USB 1.1 ports can transfer data to and from a USB data acquisition module at up to 12 Mbps. This full-speed rate is useful for data streaming applications and supports data acquisition rates of up to 400 kHz. For high-performance applications, ensure that your PC has a high-speed USB 2.0 port. With USB 2.0, you can transfer data between the PC and your USB data acquisition module at up to 480 Mbps. This increased bandwidth allows you to perform multiple I/O operations simultaneously at throughput rates up to 500 kHz in each direction, similar to PCI measurement systems.
• Cost savings: Many USB data acquisition modules include removable terminal blocks or BNC connectors that conveniently handle all user I/O connections. This design is not only convenient, but cost effective, since you don't have to purchase optional screw terminal accessories.
• Portable: USB data acquisition modules are compact and portable, allowing you to move even the most sophisticated test and measurement applications out of the lab and into the field.
• Easily expandable: Using low-cost expansion hubs and USB cables, you can connect up to 127 data acquisition modules to a single USB port.
• Hot-swappable: USB data acquisition modules can be installed or removed while the computer is running. Just plug the device in, use it and unplug it when you're done. There's no need to power down your PC. Because the USB module is self-enumerating and self-identifying, the device driver is dynamically loaded when the module is plugged in and dynamically unloaded when the module is unplugged.
• Simple power connections: USB data acquisition modules can be powered either directly by the bus or through a simple connection to an external power source. Low-powered modules draw less than 100 mA at 5 V and use the power supplied by the USB cable. Self-powered modules draw up to 500 mA at 5 V and use their own power supplies.

Potential dangers of using USB for test and measurement

While USB offers many benefits, not all USB data acquisition modules are created equal. There are hidden dangers that, depending on your application and the design of your USB module, can cause catastrophic results.

Unlike PCI boards, which have short substantial ground systems into the backplane of the PC, USB modules have a long ground connection (up to five metres) and active circuitry at both ends. If your module is not designed properly, this can cause system lock-ups, erratic performance and electromagnetic transients — significant problems for noise-sensitive measurements.

Before choosing a USB data acquisition module, consider your application:

1. Will your data acquisition module be susceptible to electrostatic discharge (ESD), lightning or power surges from motors, switching devices or other equipment?
2. Are you dealing with voltages that have different ground potentials?
3. Will the module be operated in a benign environment?

If you answered "yes" to questions 1 or 2 above, ensure that your system includes isolation. Isolation protects your PC from damage and preserves the integrity of your data by physically separating the electrical connections between circuits, thereby limiting potentially harmful voltage or current from flowing through your system. You can provide isolation either by adding signal conditioning accessories to your system, which can be expensive, or by choosing an isolated USB data acquisition module from the start.

If you answered "yes" to question 3, a non-isolated module might be the right choice for you.

Let's look at these application environments in more detail and explore the role of isolation in each case.

Case one — ESD, lightning or power surges

Figure 1 shows a typical application scenario, where a sensor is measuring a voltage from a device under test. The sensor is connected to a USB data acquisition module, which in turn is connected to the PC.

Figure 1: Typical application set-up.

ESD, lightning and power surges generate sudden transient over-voltages that, even if short-lived, can damage the electronic components in your entire system. If your USB data acquisition module is non-isolated (see Figure 2), the current that is generated by these types of events backs up through the entire system, eventually reaching and potentially damaging your PC and other system components.

Figure 2: Non-isolated USB data acquisition modules during ESD, lightning or power surges can potentially damage your system and cause data inaccuracies.

Non-isolated modules from some data acquisition vendors actually lock up the entire system in response to transient voltages, necessitating a system reboot. In test and measurement applications, this kind of behaviour is unacceptable.

In contrast, isolated modules (shown in Figure 3) dissipate harmful current across the module's ground plane, protecting your entire system.

Figure 3: Isolated USB data acquisition modules during ESD, lightning or power surges protect your pc and preserve signal integrity.

Even if the transient voltage that is introduced is small enough not to damage your system, be aware that your data can contain large errors, particularly at high resolutions. For example, if you are using a USB module with 16-bit resolution to measure a signal in the ±10 V range, the LSB value is 0.31 mV (see Table 1). Therefore, if the module is non-isolated and a transient voltage occurs in the electrical system, your data could be off by hundreds of millivolts. Even in static environments, your data could be off by tens of millivolts — huge inaccuracies when you are measuring low-level signals. As you can see, if you need high-accuracy, low-noise measurements, isolation is critical.

Table 1: Voltage ranges at different resolutions

Voltage range	Gain	16-bit resolution (LSB value)	12-bit resolution (LSB value)
±10 V	1	0.31 mV	4.88 mV
±5 V	2	0.15 mV	2.44 mV
±2.5 V	4	0.08 mV	1.22 mV
±1.25 V	8	0.04 mV	0.61 mV

Case two — different ground potentials

Single-ended analog inputs are non-isolated inputs that are referenced to earth ground. In non-isolated systems, even digital I/O signals are connected to the same ground. If your system under test shares the same ground as your USB data acquisition module (by virtue of being connected to building's power system), the difference in ground potential between the two devices can be substantial — more than 100 mV. Fast switching currents must then travel down the five-metre USB cable to the PC.

Depending on how you connect single-ended inputs to the module, you can introduce ground loop errors that, when added to your signal and other ground potentials across up to five metres of USB cable, can not only provide highly inaccurate measurements, but can also damage your system. Figure 4 shows an example of improperly connecting single-ended inputs.

Figure 4: Improperly connecting single-ended inputs can damage your system and result in inaccurate measurements.

A better connection scheme for single-ended inputs that reduces ground loop errors is shown in Figure 5.

Figure 5: Properly connecting single-ended inputs.

For the most accurate measurements, use differential inputs (shown in Figure 6). Differential inputs are isolated inputs because they are referenced to a ground reference point that is not connected to earth ground. As a result, they eliminate common-mode voltage errors that can occur when differences in ground potentials exist.

Therefore, if you are measuring low-level signals, signals where noise is a significant part of the measurement, or if common-mode voltage exists, ensure that your USB data acquisition module provides differential input connections.

Figure 6: Differential inputs are isolated and eliminate common-mode voltage errors by providing a reference ground source not tied to earth ground.

Case three — benign conditions

In benign environments, where transient electrical spikes and different ground potentials do not exist, non-isolated modules are a good, less expensive choice. A laptop that runs off battery power is a good example of a benign environment. In non-isolated systems, the PC is tied directly to the ground system of the sensor, so your measurements will be accurate as long as no noise or other errors are added to your voltage source.

To ensure that no measurement errors are introduced, it is critical that your module provides high input impedance. Assume, for example, that the source impedance of your analog signal is 1 kΩ (see Figure 7). If your module provides only 144 kΩ of input impedance, as is the case with modules from many vendors, your measurements could be off by 0.69%. Choosing a module with high input impedance, however, minimises these measurement errors.

Figure 7: High input impedance is a critical specification in non-isolated modules.

Summary

The benefits of USB for test and measurement applications are many. But, before choosing a USB data acquisition module, consider your application. If transient voltages or differences in ground potentials exist, protect your PC and preserve the integrity of your signal by choosing an isolated USB data acquisition module. If, on the other hand, you are operating in benign conditions, where transient electrical spikes and different ground potentials do not exist, choose a non-isolated module with high input impedance for error-free measurements.

Total Turnkey Solutions
www.turnkey-solutions.com.au
Data Translation, Inc
www.datatranslation.com