Selecting Aperture Time for DC Measurements
The aperture time shapes the noise rejection of the measurement. Shorter aperture time yields a wider measurement bandwidth at the expense of resolution. Sensitivity to source noise increases with shorter apertures. Conversely, larger aperture times quiet noisy sources and yield higher resolution measurements. Refer to Noise for more information.
Selecting aperture times equal to multiples of power line frequency helps to reject these frequencies, when DC Noise Rejection is set to Normal. An aperture set to 1 powerline cycles (1 PLC equals 16.67 ms for 60 Hz powerline or 20 ms for 50 Hz) is the minimum that provides this line rejection.
Apertures of >100 ms can be used for measuring very high resistance values, when sensitivity to powerline frequency noise pickup is inevitable. Coupling the aperture time with DC Noise Rejection set to High Order provides extremely high noise rejection (>100 dB). Long apertures of >100 ms do not significantly improve resolution beyond 6½ digits but can provide better rejection to externally generated noise.
For voltage measurements with 7-digits resolution, average 4 or more 6½-digit Auto Zero measurements. You can average multiple measurements by setting the Number of Averages property to a value greater than one. Averaging with Auto Zero enabled circumvents the effects of low–frequency, self-generated noise and yields the highest resolution performance level. Having more averages also reduces external noise, but the tradeoff is speed.
Selecting apertures for AC is discussed in AC Voltage Measurements.