Rarely do common-mode voltages consist of only a DC level. Most sources of common-mode voltage contain an AC component in addition to a DC offset. Noise is inevitably coupled onto a measured signal from the surrounding electromagnetic environment. This is particularly troublesome for low-level analog signals passing through the instrumentation amplifier on a DAQ device.
Sources of AC noise may be broadly classified by their coupling mechanisms – capacitive, inductive, or radiative. Capacitive coupling results from time-varying electric fields, such as those created by nearby relays or other measurement signals. Inductive or magnetically coupled noise results from time-varying magnetic fields, such as those created by nearby machinery or motors. If the electromagnetic field source is far from the measurement circuit, such as with fluorescent lighting, the electric and magnetic field coupling is considered combined electromagnetic or radiative coupling. In all cases, a time-varying common-mode voltage is coupled onto the signal of interest, most often in the range of 50-60 Hz (power-line frequency).
An ideal measurement circuit has a perfectly balanced path to both the positive and negative terminals of an instrumentation amplifier. Such a system would completely reject any AC-coupled noise. A practical device, however, specifies the degree to which it can reject common-mode voltage with a common-mode rejection ratio (CMRR). The CMRR is the ratio of the measured signal gain to the common-mode gain applied by the amplifier, as noted by the following equation:
Choosing a DAQ device with a better CMRR over a broader range of frequencies can make a significant difference in your system’s overall noise immunity. For example, Figure 3 shows the CMRR for a low-cost M Series device compared with that of an industrial M Series device.
Figure 3: The NI 6230 provides a much higher CMRR than the NI 6220 relative to earth ground.
At 60 Hz, NI 6230 industrial M Series devices have 20 dB greater CMRR than NI 6220 low-cost M Series devices. This is equivalent to a 10 times better attenuation of 60 Hz noise.
Any application may benefit from rejecting 60 Hz noise. However, those with large rotating machinery or motors require noise immunity at higher frequencies. At 1 kHz, NI 6230 devices reject noise 100 times better than NI 6220 devices, making them ideal for industrial applications.