Resolution: An analog-to-digital converter (ADC) converts an analog voltage to a digital number. The digital number represents the input voltage in discrete steps with finite resolution. ADC resolution is determined by the number of bits that represent the digital number. An n-bit ADC has a resolution of 1 part in 2n. Thus, 12 and 16 bit resolutions are as follows:
•12-bit resolution: 1 part in 4096 (212), corresponding to 2.44 mV in a 10 V range.
•16-bit resolution: 1 part in 65,536 (216), corresponding to 0.153 mV in a 10 V range.
5.6System Noise
Laboratory and industrial environments often have multiple sources of electrical noise. An AC power line is a source of 50/60 Hz noise. Heavy equipment (air conditioners, elevators, pumps, etc.) can be a source of noise, particularly when turned on and off. Local radio stations are a source of high-frequency noise, and computers and other electronic equipment can create noise in a multitude of frequency ranges. Thus, an absolute noise-free environment for data acquisition is not realistic. Fortunately, noise-reduction techniques such as averaging, filtering, differential voltage measurement, and shielding are available to reduce noise to an acceptable level.
5.7 Averaging
Certain acquisition programs apply averaging after several samples have been collected. Depending on the nature of the noise, averaging can reduce noise by the square root of the number of averaged samples. Although averaging can be effective, it suffers from several drawbacks. Noise in measurements only decreases as the square root of the number of measurements—reducing RMS noise significantly may require many samples. Thus, averaging is suited to low- speed applications that can provide many samples.
NOTE:
Only random noise is reduced or eliminated by averaging.
Averaging does not reduce or eliminate periodic signals.
5.8 Analog Filtering
A filter is an analog circuit element that attenuates an incoming signal according to its frequency. A lowpass filter attenuates frequencies above the cutoff frequency. Conversely, a high-pass filter attenuates frequencies below the cutoff. As frequency increases beyond the cutoff point, the attenuation of a singlepole, low-pass filter increases slowly. Multi-pole filters provide greater attenuation beyond the cutoff frequency but may introduce phase (time delay) problems that could affect some applications.
