Isolation specifications are often overlooked but that doesn’t diminish their necessity in many applications. Isolation, specifically channel-to-earth ground isolation, is important for three reasons: signal integrity, safety, and instrument protection. Without detailed specifications, the only thing that isolation can tell you is whether ground loops will be broken, giving you better signal integrity. While this information is important, you should also look for a common-mode rejection ratio (CMRR) to earth ground to improve your signal integrity. CMRR details how well the device can reject common-mode signals or unwanted voltages that are common to both input terminals. Devices with isolation not only reject ground loops but also improve signal integrity by having a higher CMRR to earth ground.
For more information on isolation and measurement quality, see Isolation Technologies for Reliable Industrial Measurements.
According to Electrical Safety Foundation International, electrical hazards cause more than 300 deaths and 4,000 injuries per year. Isolation specifications can play an important role when dealing with safety and avoiding accidents. It can be used to separate high-voltage circuits from safety extra-low voltage (SELV) or user-touchable circuits. Especially in the case where the entire system is ungrounded, isolation helps prevent the SELV circuits from becoming energized when hazardous voltages are placed on the device. Because it would be impossible to ensure safety for every voltage level, isolation specifications can help you to specify the situations and how much voltage is safe to use with the device. This includes what voltages are safe and the measurement category of the voltage.
For more information on safety and isolation, see Safety Isolation Protects Users and Electronic Instruments[EDN].
Finally, you can use isolation to protect your instrumentation when taking measurements with high common-mode voltages present. The range of an instrument is bounded by its amplifiers and converters, which are bounded by their power supplies and references, which are attached to the local ground. For a non-isolated instrument, the local ground is the system/earth ground. So the devices input range is in a window around earth ground. However, in an instrument with channel-to-earth isolation, the instrument’s local ground is not attached to earth ground. An isolated instrument therefore has a measurement range that is in a window around its isolated-local ground, which could float at a much different voltage than earth ground.
For a device with channel-to-channel and channel-to-earth isolation, this means that each channel has its own local ground that can float free of both earth ground and the other channels. This can allow the measurement of several tiny mV signals such as thermocouples each sitting at its own unique common-mode voltage. For example, when measuring temperatures of cells in an electric vehicle’s battery stack, a thermocouple instrument might have an input range of only 100 mV, but channel-to-channel isolation would permit each thermocouple to sit on top of the incremental 3.5 V per cell. The limit to how high the common voltages could go (or how many consecutive cells you could measure with that instrument) is then limited not by the input ranges of the amplifiers nor by the power supplies, but rather by the channel-to-earth ground or channel-to-channel isolation voltage (which is usually larger than the input range).
Figure 1: Channel-to-Earth Ground Isolation With Channel-to-Channel Isolation
As with safety, it would be impossible to create a device that could withstand unlimited common-mode voltage. Therefore it is critical that detailed isolation specifications be given to understand how much common-mode voltage your system can handle. These specifications give you the ability to know what window your device can measure in and, in turn, help you to avoid error and potential instrument damage.