Aligning Moving Average Windows with Converters to Eliminate Aliasing Artifacts
Aligning moving average window lengths to match converter sample periods places exact spectral nulls over periodic interference to eliminate aliasing artifacts.
Powerline Rejection
Interference Suppression
Noise cancellation in high-precision measurement instruments must target the specific electromagnetic frequencies radiated by mains power grids. Achieving high powerline rejection prevents the alternating current fields of fifty or sixty hertz from corrupting the delicate analog signals of sensitive sensors. This capability is measured in decibels, and is critical for instruments operating near power cables.
Filter Design
Analog notch filters or digital comb filters are designed to attenuate the specific frequency of the powerline and its harmonics. When the powerline rejection is implemented digitally, the filter must have a narrow bandwidth to avoid distorting adjacent signal frequencies. This precision ensures that the primary signal remains intact while the powerline noise is suppressed.
Common-Mode Signal
Differential amplifiers utilize high common-mode rejection to eliminate powerline noise that appears equally on both signal wires. The quality of this rejection depends on the matching of the input impedances and the gain stages of the amplifier. In field installations, shielded twisted-pair cables are used to ensure that the noise is coupled equally into both wires, allowing the amplifier to subtract the noise and output a clean signal.
Metrological Verification
Calibration procedures test the rejection ratio by injecting a simulated powerline signal into the measurement channel. This test verifies that the instrument meets its specifications before shipment.