Isolation boosts safety and integrity

In some measurement applications, you may find high voltages that interfere with an instrument's ability to properly measure millivolt-level or volt-level signals. At worst, a high voltage such as a line voltage can pose a safety hazard to the people operating the equipment. At best, unwanted voltages can damage equipment or produce unacceptable measurement errors. Your measurement system, therefore, must protect you from hazardous voltages and reject unwanted voltages that can compromise signal integrity.

Isolating a signal source from a measurement instrument can keep high voltages away from people and equipment. In addition, isolation devices can reject unwanted voltages from interfering with your measurements.

Unwanted voltages often appear because of a difference in potential between two "grounds" in a building's electrical wiring. Figure 1 shows a measurement system that contains a voltage between the ground connections of a sensor and an instrument. Because the unwanted voltage appears with equal magnitude on both of the instrument's differential inputs, it's called a common-mode voltage (V_cm) (Ref. 1). Common-mode voltages often raise a signal's level high above the ground potential of an instrument.

Figure 1. A common-mode voltage can appear across both instrument inputs, which can pose a safety hazard, damage the instrument, or cause measurement errors.

Should V_cm reach AC mains potential, it not only will damage the instrument, it will create a shock hazard to equipment operators. Even if V_cm doesn't reach hazardous levels, voltages greater than 35 V above an instrument's ground potential can still overload and damage an instrument's input circuits.

In applications such as battery measurements, in which the signal source and measurement equipment reside close to each other, there is no difference in ground potential. These in-circuit measurements may still require measurements not referenced to ground, though. For example, you may need to measure current by measuring the voltage across a shunt resistor, or in the case of a battery test, you may require several batteries in series and need to measure the voltage at any battery. If the shunt resistor or battery lacks a connection to ground, you need to measure the small voltage—perhaps millivolts for a shunt resistor—while the resistor or battery signal sits volts or tens of volts above ground.

You can't add a ground connection to an ungrounded circuit component for a test, but perhaps you can add or move grounds if your system uses sensors. You can minimize V_cm by removing the local ground from the sensor's power supply and running a ground wire directly to the instrument's ground, thus creating a single reference point. In many applications, though, it's impractical to run a separate ground wire for each sensor—either the sensor sits too far away from the instrument, or else the system uses numerous sensors. Furthermore, safety regulations often require local ground connections.

Ground wires can pick up magnetic fields and convert them into current (Ref. 2). That current, combined with wire resistance, can create a common-mode voltage that can cause measurement errors—unless your measurement system adequately rejects it. When you can't connect your sensor and instrument to the same ground reference, or when you need to measure a voltage that lacks a ground reference, you can use isolation. With isolation, you remove any direct electrical connection between your signal source and your instrument, and you break ground loops that can pick up interference.

You can choose any of several options for isolating a sensor or circuit component from an instrument. The most common method incorporates an isolation amplifier, which uses a transformer to transfer signals without a hard-wired connection. See "Inside an isolation amplifier" (below) for a description of how these devices isolate signals. Other options include optoisolators, which work best on digital signals, and capacitive couplers. I'll concentrate on isolation amplifiers, their characteristics, and their applications.

Isolation amplifiers provide several benefits. They prevent most high voltages from reaching an instrument's low-voltage inputs, protecting both the equipment and the people who operate it. Isolation amplifiers also break ground loops, thus removing common-mode voltages from a measurement circuit. They can amplify or attenuate input signals of many ranges to create signals with a common voltage range, and they also can provide isolated DC power for active sensors.

Figure 2 shows an isolation amplifier that's part of a measurement system. The sensor's output represents a normal-mode voltage, or the signal you want to measure. With an isolation amplifier in the system, V_cm now represents the voltage between the signal-carrying wires and a local ground. The isolation-mode voltage (V_im)—the difference in ground potential—resides across the isolation barrier.

Figure 2. An isolation amplifier places a barrier between a sensor and an instrument.

If V_im gets too high, the isolation barrier can break down. To prevent breakdown, you must use an isolation amplifier capable of withstanding the voltages in the test environment. When specifying an isolation amplifier, check the device's working voltage—the continuous AC or DC voltage that the device can withstand across its isolation barrier.

Many isolation amplifiers offer a working-voltage (also called "continuous voltage" or "transformer isolation") rating of 1500 V, well in excess of any line voltage. An isolation amplifier's data sheet, though, may specify the test voltage used in a compliance test rather than a working voltage. Test voltages are considerably higher (typically 5X) than working voltages, but the test voltage applies for a 1-s duration only.

Don't confuse an isolation amplifier's working-voltage rating with its input-protection rating, either. Working voltage refers to the voltage across an isolation barrier. Input protection, which typically runs from 240 V to 300 V, refers to the maximum voltage you can place across an isolation amplifier's input terminals (normal-mode voltage) without damaging the device.

Once you determine that an isolation amplifier's working voltage and input-protection rating meet your needs, look at the common-mode rejection (CMR) spec. CMR, expressed in decibels, specifies how well an isolation amplifier attenuates common-mode voltages that can interfere with measurements on normal-mode voltages.

A typical isolation amplifier will attenuate common-mode signals by as much as 140 dB to 150 dB from DC to 60 Hz, although CMR will vary depending on the amplifier's gain (more gain produces higher CMR). In contrast, a typical data-acquisition card configured with differential inputs attenuates common-mode voltages by about 80 dB to 100 dB.

Manufacturers of isolation amplifiers design their devices for best CMR at power-line frequencies, 50 Hz or 60 Hz. But common-mode voltages at higher frequencies can pass through an amplifier more easily due to capacitive coupling across an isolation barrier (Ref. 3). Even if an isolation amplifier contains filters that attenuate the high-frequency noise, the device's circuits can rectify those noisy signals, creating offset errors in the device. To minimize errors caused by high-frequency signals. Place a capacitor across your sensor's signal wires. The value depends on the frequency of your signal (Ref. 4).

While protection from hazardous voltages and rejection of common-mode voltages form two important considerations for selecting an isolation amplifier, you must look at gain and input range, too. When specifying an isolation amplifier for a data-acquisition system, you usually use a module rather than design your own isolation board using an IC. Isolation amplifier modules often come in many models, each with a different gain. Those gains result in modules with different input and output ranges.

One isolation amplifier module might accept voltages of ±10 mV full scale while another can accept up to ±40 V full scale. The module will use internal resistors that set the isolation amplifier's gain, providing you with the specified input range and output range. If you use an isolation amplifier IC, you'll need to supply your own gain and offset circuits.

Output voltages for isolation-amplifier modules usually cover 0–5 V or ±5 V, so they can convert signals of many amplitude ranges into signals with a common range. If you use a scanning digitizer, you can program all the digitizer's channels to the same input-voltage range, which will increase scanning speed.

Isolating a signal source often requires more than just an isolation amplifier in the signal path. If you use a passive sensor, you need only isolate the signal path. If your sensor requires power, you need to isolate the sensor's power from the instrument's power, or you'll still get ground loops. Many isolation amplifiers contain small isolated DC-DC converters for this purpose.

Many data-acquisition equipment makers supply isolation amplifiers, and they often provide a backplane card where you can attach these modules and a ribbon cable that connects the modules to an instrument. The backplane card also provides terminal strips to which you can connect signal wires.

Isolation amplifiers will protect you and your equipment from hazardous voltages. By breaking ground loops, isolators also minimize susceptibility to common-mode voltages and can improve measurement performance.