Dynamic Signal Acquisition Concepts

Nyquist Frequency and Nyquist Bandwidth

Any sampling system, such as an ADC, is limited in the bandwidth of the signals it can measure. Specifically, a sampling rate of fs can represent only signals with frequencies lower than fs/2. This maximum frequency is known as the Nyquist frequency. The bandwidth from 0 Hz to the Nyquist frequency is the Nyquist bandwidth.

Noise

Notice Electromagnetic interference can adversely affect the measurement accuracy of the DSA products described in this document. The inputs and outputs of these products are not connected to chassis ground for functional reasons. Therefore, the outer conductor of any connected coaxial cable is not connected to chassis ground and the outer conductor will not act as a shield for unwanted noise. The shield can act as an antenna to transmit noise into the environment or receive noise from the environment that could affect measurement accuracy. To ensure proper shielding effectiveness of connected coaxial cables, the outer conductor must be directly connected to chassis or earth ground at the load end of the cable. In addition, snap-on ferrite beads or other remedial measures may be required to prevent unwanted emissions or immunity. Refer to the PXIe-4467 Specifications for more information about EMC performance.

DSA devices typically have a dynamic range of more than 110 dB. Several factors can degrade the noise performance of input channels, such as noise picked up from nearby electronic devices. DSA devices work best when kept as far away as possible from other plug-in devices, power supplies, disk drives, and computer monitors. Cabling is also critical. Use well-shielded coaxial or floating cables for all connections. Route the cables away from sources of interference such as computer monitors, switching power supplies, and fluorescent lights. Physical motion or deformation can induce noise on sensitive analog cables. Use a transducer with a low output impedance to minimize system susceptibility to external noise sources and crosstalk.

You can reduce the effects of noise on your measurements by carefully choosing the sample rate to maximize the effectiveness of the anti-alias filtering. Computer monitor noise, for example, typically occurs at frequencies between 15 kHz and 65 kHz. If the signal of interest is restricted to below 10 kHz, for example, the anti-alias filters reject the monitor noise outside the frequency band of interest, and a sampling rate of at least 21.6 kS/s guarantees that any signal components in the 10 kHz bandwidth of interest are acquired without aliasing and without being attenuated by the digital filter.

When possible, use the differential configuration to minimize the effect of any noise produced by ground currents in the chassis and common-mode noise. If you have particularly noisy AC power, consider external filtering, such as a line conditioner or an uninterruptible power supply.