There are many issues to consider when measuring high voltage. When specifying a data acquisition (DAQ) system, the first question you should ask yourself is whether or not the system will be safe. Making high voltage measurements can be hazardous to your equipment, to the unit under test, and even to you and your colleagues. To ensure that your system is safe, you should provide an insulation barrier between the user and hazardous voltages with isolated measurement devices.
Isolation is a means of physically and electrically separating 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. By providing electrical isolation, you can break ground loops, increase the common mode range of the DAQ system, and level shift the signal ground reference to a single system ground. 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 that the user may come in contact with.
Incorporating isolation into a data acquisition system has three primary functions: preventing ground loops, rejecting common-mode voltage and providing safety.
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. The local ground of your system can be several volts above or below the ground of the nearest building, and nearby lightning strikes can cause the difference to rise to several hundreds or thousands of volts. This additional voltage itself can cause significant error in the measurement, but the current that causes it can couple voltages in nearby wires as well. These errors can appear as transients or periodic signals. For example, if a ground loop is formed with 60Hz AC power lines, the unwanted AC signal appears as a periodic voltage error in the measurement.
When a ground loop exists, the measured voltage, Vm, is the sum of the signal voltage, Vs, and the potential difference, Vg, which exists between the signal source ground and the measurement system ground (as shown in Figure 1 below). This potential is generally not a DC level; thus, the result is a noisy measurement system often showing power-line frequency (60 Hz) components in the readings.
Figure 1. A Grounded Signal Source Measured with a
Ground-Referenced System Introduces Ground Loop
To avoid ground loops, ensure that there is only one ground reference in the measurement system, or use isolated measurement hardware. Using isolated hardware eliminates the path between the ground of the signal source and the measurement device, thus preventing any current from flowing between multiple ground points.
An ideal differential measurement system responds only to the potential difference between its two terminals, the (+) and (-) inputs. The differential voltage across the circuit pair is the desired signal, yet an unwanted signal may exist that is common to both sides of a differential circuit pair. This voltage is known as common-mode voltage. An ideal differential measurement system will completely reject, rather than measure, the common-mode voltage. Practical devices, however, have several limitations, described by parameters such as common-mode voltage range and common-mode rejection ratio (CMRR), which limit this ability to reject the common-mode voltage.
The common-mode voltage range is defined as the maximum allowable voltage swing on each input with respect to the measurement system ground. Violating this constraint results not only in measurement error, but also in possible damage to components on the board.
Common mode rejection ratio describes the ability of a measurement system to reject common-mode voltages. Amplifiers with higher common-mode rejection ratios are more effective at rejecting common-mode voltages. The common-mode rejection ratio (CMRR) is defined as the logarithmic ratio of differential gain to common mode gain.
CMRR (dB) = 20 log (Differential Gain/Common-Mode Gain). (Equation 1)
Common-mode voltage is shown graphically in Figure 2. In this circuit, CMRR in dB is measured as 20 log Vcm/Vout where V- = Vcm.
CMRR Measurement Circuit
In a non-isolated differential measurement system, an electrical path still exists in the circuit between input and output. 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.