Advantages and potential dangers of using USB for measurement applications

Advantages and potential dangers of using USB for measurement applications

Introduction: USB has many advantages for test and measurement applications, but before selecting a USB data acquisition module, carefully consider the target application. If transient voltage or ground potential differences exist, selecting a USB data acquisition module with isolation measures can protect the PC and maintain the integrity of signal data. This article analyzes in detail the advantages and potential dangers of using USB data acquisition modules, introduces a method to eliminate this potential danger - isolation, and understands the different effects of isolation through actual application cases.

Because of its ease of use, USB has become one of the fastest growing buses in the computer and electronics industries. For test and measurement applications, USB data acquisition modules offer several significant advantages. But beware, depending on the application, they may also contain some potential dangers that may even lead to catastrophic consequences.

Advantages of Using USB for Test and Measurement

USB has the following advantages, making it an easy choice for users to develop test and measurement applications.

1. True plug-and-play: Simply connect the data acquisition module to the PC's USB port using a standard, low-cost cable. When the module is plugged in, the PC automatically recognizes the module and installs the necessary software to operate the module. This connection method greatly reduces startup time.

2. No longer do you need to open the PC case to add boards, configure jumper switches and interrupt settings, search for the right device driver, or reboot the system. Simply connect the sensor to the module and forget it. Within minutes, you are continuously acquiring data, temperature, pressure, sound, or whatever information you need.

3. Less affected by PC noise: USB data acquisition modules bring performance advantages to noise-sensitive measurement applications. Because USB cables are usually 1 to 5 meters long, the I/O circuits are farther away from the computer's electromagnetically noisy motherboard and power supply, and closer to the sensors to be measured.

4. Full-speed and high-speed transfer rates: Computers with USB1.1 ports can input and output data from the USB data acquisition module at transfer speeds up to 12Mbps. This full-speed rate is useful for data streaming applications and can support data acquisition rates up to 400KHz.

   For high-performance applications, it is important to ensure that the PC has a high-speed USB 2.0 port. With USB 2.0, data can be transferred between the PC and the USB data acquisition module at speeds of up to 480 Mbps. This increased bandwidth allows multiple I/O operations to be performed simultaneously, with traffic rates of up to 500 kHz per channel, similar to PCI measurement systems.

5. 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 also saves money because you no longer need to purchase optional screw-on terminal accessories.

6. Portability: USB data acquisition modules are small and easy to carry, allowing users to take even the most complex test and measurement applications out of the lab and into the field.

7. Easy to expand: Use low-cost expansion hubs and USB cables to connect up to 127 data acquisition modules to one USB port.

8. Hot-swappable: The USB data acquisition module can be installed or removed while the computer is running. Simply plug in the device when in use and unplug it when the work is done, without shutting down the computer. Because the USB module can count and identify itself, when the module is plugged in, the device driver is automatically loaded; when the device is unplugged, the device driver is automatically unloaded.

9. Simple power connection: USB data acquisition modules can be powered via the USB bus or by simply connecting to an external power source. Low-power modules draw less than 100mA at 5V and can be powered via the USB cable. Self-powered modules draw up to 500mA at 5V and use their own power supply.

Potential Dangers of Using USB for Test and Measurement

Although USB offers many advantages, not all USB data acquisition modules are designed in the same way. Depending on the application, there may be potential dangers and a USB module design may have catastrophic consequences.

Unlike PCI boards that have a true ground system with a short distance to the PC backplane, USB modules have long ground connections (up to 5 meters) at both ends of the cable and active circuits. If the module is not designed properly, this can cause system lockup, unstable performance, and electromagnetic interference, which is a major problem for noise-sensitive measurements.

Before selecting a USB data acquisition module, consider the following questions for your target application:

1. Is the data acquisition module susceptible to electrostatic discharge (ESD), lightning, or power surges from motors, switching gear, or other equipment?

2. Does the application involve voltages with different ground potentials?

3. Will this module work in a good environment?

If the answer to question 1 or 2 above is yes, you need to make sure your system has isolation. Isolation protects your PC from damage and your data integrity by physically isolating the electrical connections between circuits, which limits the amount of potentially harmful voltage or current that can pass through your system. You can also provide isolation by adding signal conditioning accessories to your system, which can be expensive, or by choosing an isolated USB data acquisition module from the beginning.

Let's take a closer look at these application environments and understand the role of isolation in each case.

Case 1: ESD, Lightning, or Power Surge

Figure 1 shows a typical application scenario where a sensor is measuring the voltage of a device under test. The sensor is connected to a USB data acquisition module on one end and to a PC on the other end.

ESD, lightning, and power surges generate sudden, transient overvoltages that, even for a short time, can damage electronic components throughout the system. If the USB data acquisition module is not isolated (see Figure 2), the current generated by these events will flow back through the entire system and eventually reach the PC and may damage the PC and other system components.

Some data acquisition vendors offer non-isolated modules that actually lock up the entire system when an overvoltage transient occurs, forcing the system to be restarted. This behavior is unacceptable in measurement applications.

In contrast, an isolated module (shown in Figure 3) discharges harmful currents through the module's ground plane to protect the entire system.

Even if the transient voltage introduced is small enough not to damage the system, be careful that your data may contain large errors, especially at high resolution. For example, if you use a USB module with 16-bit resolution to measure a signal in the ±10V voltage range, the LSB value is 0.31mV (see Table 1). If this module is not isolated and a transient voltage appears in the electrical system, the data may be off by hundreds of millivolts. Even in a static environment, your data may be off by tens of millivolts. This can be a huge error when measuring low-level signals.

Isolation is critical if high-precision, low-noise measurements are required. All of Data Translation's USB modules, from the DT9801 series to the DT9834 series, provide up to 500V of electrical isolation. Electrical isolation converts the energy in the input signal into an output signal that flows away on the module's ground plane. As a result, your computer remains safe and your measurements are more accurate.

Taking the DT9801 module as an example, electrical isolation up to 500V is provided by using the following components:

1. Transformer: Converts energy from fast clock signals without delay;

2. Optical isolators: convert energy from slower control signals with delays of tens of milliseconds;

3. Differential capacitive coupling: Converts energy from slow data channels with 1 ms delay.

Case 2: Different ground potentials

Single-ended analog inputs are non-isolated inputs referenced to earth ground. In non-isolated systems, even digital I/O signals are connected to the same ground. If the system under test and the USB data acquisition module share the same ground (via connection to the building's power system), the difference in ground potential between the two devices can be substantial (more than 100mV). Fast switching currents must flow along the 5-meter USB cable to the PC.

Depending on how you connect the single-ended input to the module, you can introduce ground loop errors, which not only lead to highly inaccurate measurements, but can also damage the system under test when adding signal and other ground potentials in USB cables up to 5 meters long. Figure 5 shows an example of an improper single-ended input connection.

Figure 6 shows a better single-ended input connection scheme that can reduce ground loop errors.

For the most accurate measurements, use differential inputs (shown in Figure 7). Differential inputs are isolated inputs because the ground reference point they reference is not connected to earth ground. As a result, they eliminate common-mode voltage errors that occur when differences in ground potential are present.

Therefore, if you are measuring low-level signals where noise is a critical component of the measurement, or if common-mode voltages are present, make sure the USB data acquisition module provides differential input connections. All of Data Translation's USB modules, from the DT9801 series to the DT9834 series, provide up to 8 or 16 differential input connections to provide maximum ground loop protection.

Case 3: Good situation

In a good environment, transient electrical signal glitches and ground potential differences do not exist, so isolation is not needed. In a non-isolated system, the PC is connected directly to the sensor's ground system, so as long as no noise or other errors are added to the voltage source, the measurement will be accurate.

Although non-isolated solutions may be cheaper to purchase, test and measurement applications are rarely in good condition. So be careful, if you choose a non-isolated solution, you may incur measurable back-end costs due to inaccurate data or additional expenses caused by system failures.