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114 Electric Drives and Electromechanical Systems

the effect of noise, particularly interference from the mains supply. The solution, to this
problem is to apply an anti-alias filter which blocks frequencies higher than those of
interest.

4.1.4 Analogue-digital and digital-analogue conversion errors
Conversion of an analogue signal to a digital value involves a process of quantisation. In
an analogue-to-digital (A/D) converter, the change from one state to the next will occur
at a discrete point (the intermediate values are not considered, Fig. 4.8. The difference
between any two digital values is known as the quantisation size, V q , and it is commonly
termed the resolution of the converter. For an n-bit system the steps due to quantisation
step V q , and the subsequent error E q are equal to,
Full scale input
V q ¼ n (4.7)
2
1 Full scale input Fullscale input
E q ¼ n ¼ (4.8)
2 2 2 nþ1
The resolution is equal to the input voltage, V q , which will change the state of the
least-significant bit (LSB).
Transitions occur from one digital number to the next at integral multiples of the LSB,
giving a maximum uncertainty of one bit within the system. The resolution can only be
decreased by increasing the number of bits within the converter. A range of techniques
are used for analogue to digital conversion, including high-speed-flash (or parallel)
converters, integrating, and successive-approximation converters. It is not common to
construct a discrete system; one of the commonly available proprietary devices is usually
used in the selection of a suitable device, and consideration must be given to the device’s

FIG. 4.8 Errors introduced through the analogue to digital conversion process. The voltage being converted is the
solid line, with the digital value below. It can the noted that the ADC value is correct only at the conversion
points, and then only to the nearest digital value.