Types of High Voltage and Isolated Measurements
Industrial and process control applications often involve environments with hazardous voltages, transient signals, common-mode voltages, and fluctuating ground potentials capable of damaging measurement systems and ruining measurement accuracy.
Common-mode voltage is an unwanted signal that is common to both sides of a differential circuit pair. An ideal differential measurement system completely rejects, rather than measures, the common-mode voltage. In a nonisolated differential measurement system, an electrical path still exists in the circuit between input and output.
Figure 1. An ideal instrumentation amplifier completely rejects common-mode voltage.
Therefore, electrical characteristics of the amplifier limit the common-mode signal level that can be applied to the input. With the use of isolation amplifiers, the conductive electrical path is eliminated and the common-mode rejection ratio is dramatically increased.
Figure 2. Isolation electrically separates the instrumentation amplifier ground reference from earth ground.
Ground loops are the most common source of noise in data acquisition applications. They occur when two connected terminals in a circuit are at different ground potentials, causing current to flow between the two points. This additional voltage can cause significant error in the measurement, but the current that causes it can couple voltages in nearby wires as well.
Figure 3. A differential thermocouple measurement with a grounded signal source can create a ground loop.
These errors can appear as transients or periodic signals resulting in a noisy measurement system. Using isolated hardware eliminates the path between the ground of the signal source and the measurement device, therefore preventing any current from flowing between multiple ground points.
Figure 4. Isolation eliminates ground loops by separating earth ground from the amplifier ground reference.
When measuring high voltage, you should consider common-mode voltage and ground loops in selecting hardware for your system.
Learn more about common-mode voltages and ground loops
Designing the Right Measurement System for High Voltage and Isolated Measurements
When measuring high voltages, you should consider attenuation, isolation, and safety to build a complete and accurate system. To measure high voltages (>10 V), you need a voltage attenuator to adjust the range of the signal to the ADC range. You can find built-in attenuation up to 1,000 V in off-the-shelf signal conditioning hardware. Voltages outside of the input range require a voltage transformer to convert the signal to a lower range.
Isolation physically and electrically separates two parts of a measurement device and can be categorized into electrical and safety isolation. Electrical isolation pertains to eliminating ground paths between two electrical systems. It is important to understand the isolation topology of a device when configuring a measurement system as different topologies have associated cost and speed considerations.
The most powerful isolation topology is channel-to-channel isolation. In this topology, each channel is individually isolated from one another and from other nonisolated system components. In addition, each channel has its own isolated power supply.
In terms of speed, there are several architectures from which to choose. Using an isolation amplifier with an ADC per channel is typically faster because you can access all of the channels in parallel. A more cost-effective but slower architecture involves multiplexing each isolated input channel into a single ADC.
Another method of providing channel-to-channel isolation is to use a common isolated power supply for all of the channels. In this case, the common-mode range of the amplifiers is limited to the supply rails of that power supply, unless you use front-end attenuators.
The banking isolation topology groups several channels together to share a single isolation amplifier. In this topology, the common-mode voltage difference between channels is limited, but the common-mode voltage between the bank of channels and the nonisolated part of the measurement system can be large. Individual channels are not isolated, but banks of channels are isolated from other banks and from the ground. This topology is a lower-cost isolation solution because this design shares a single isolation amplifier and power supply.
Read a white paper about isolation
“Safety isolation” references standards that have specific requirements for isolating humans from contact with hazardous voltages. It also characterizes the ability of an electrical system to prevent high-voltage and transient voltages to be transmitted across its boundary to other electrical systems with which the user may come in contact.
While there are several safety standards concerning isolation and high voltages, test and measurement equipment typically deals with a limited set of U.S. and international standards. The International Electrotechnical Commission (IEC) created four categories to partition circuits with different levels of overvoltage transient conditions (for example, Category I, Category II, and so on)
Learn more about safety standards
NI Measurement Systems for High Voltage and Isolated Measurements
NI offers many products that provide isolation for the measurement and automation application. Most of the isolation products that NI makes fall under the standards outlined by IEC 1010-1 and UL 3111-1, which address standards for measurement, control, and laboratory use.
Figure 5. CompactDAQ offers a portable and rugged solution with options for simultaneous sampling and up to 600 V channel-to-channel isolation.
CompactDAQ offers a portable and rugged solution with options for simultaneous sampling, built-in antialias filtering, and up to 24-bit resolution across numerous measurement types. You can choose analog input modules with up to 300 Vrms input range, 600 Vrms (Category II) channel-to-channel isolation, and 2,300 Vrms transient withstand. For applications with lower isolation requirements, most C Series analog input I/O modules are bank-isolated to provide accurate analog measurements at a lower cost.
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Figure 6. The PXI platform is recommended for fast and accurate isolated voltage measurements up to 1,000 V.
The PXI platform is recommended for fast and accurate isolated voltage measurements. The PXI Express data acquisition module with integrated signal conditioning measures up to 300 V inputs with 300 Vrms (Category II) channel-to-channel isolation. It also features an ADC per channel for simultaneous measurements with 250 kS/s per channel sample rates. M Series data acquisition (DAQ) devices offer additional solutions for lower isolation requirements and include analog output, 5 V TTL digital I/O, and counter/timers for multifunction data acquisition.
For extended input voltages, a PXI digital multimeter (DMM) can be used to measure signals from ±10 nV to 1,000 V with sampling rates up to 1.8 MS/s. The DMM can be combined with a PXI switch to form a multichannel, high-voltage data acquisition system with up to 500 VDC common-mode isolation.
View specs and pricing for PXI devices with channel-to-channel isolation
Figure 7. The SCXI platform provides the most cost-effective solution for high-channel-count measurements over 300 V.
The SCXI platform provides the most cost-effective solution for high-channel-count applications requiring isolated measurements over 300 V. High-voltage SCXI modules can withstand transient voltages up to 4,000 V and connect to customized accessories for inputs up to 600 V (Category II) and 1,000 V (Category I) with channel-to-channel isolation. Channels have independently configurable amplification, filtering, and excitation sources to fit a wide variety of sensors and signal input requiring isolation.
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