of the values. At 0 miles, the analog signal is 2 volts
peak to peak and the digital value is all ZEROS. For an
indication of 5 miles, the analog signal increases to 7
volts peak to peak and the digital value now has bits
20 and 22 set. At 20 miles, the ac signal has increased
to 22 volts peak to peak and bits 22 and 24 are set in the
digital value. Finally when the maximum value is
reached, the ac signal is 34 volts peak to peak and the
digital value has all its bits set.
You should be aware that the values we have
covered are extremely limited compared to the
capabilities of most analog and digital devices. Much
greater accuracy and ranges are commonly
encountered; however, the basic fundamentals you have
just learned will apply.
THE ANALOG-TO-DIGITAL CONVERTER
An analog-to-digital converter is a device or
component of a larger device that receives an analog
signal and converts it into a digital quantity with a given
accuracy and resolution.
The analog signal input is
compared to a given reference signal, and the
difference between signals is used to compute the
digital quantity indicated by the analog signal. The
reference signal is normally equivalent to the maximum
value of transmitted data.
The basic analog-to-digital conversion process can
be divided into a series of operations. Each operation
performs a specific task in the conversion process. The
analog-to-digital conversion operations are sampling,
quantization, and encoding.
Sampling
Sampling is the first operation that takes place in an
analog-to-digital conversion. Basically, the inputted
analog signal is sampled or tested repeatedly over a
period of time.
This is done to determine the
characteristic that contains the analog quantity, such as
the signal’s amplitude. A constantly varying input must
be sampled at a much higher frequency than its own to
ensure the accuracy of the conversion. Figure 13-2
shows a pulsed sampling of an ac signal. For each
sample taken, a voltage level is determined. By
comparing the voltages detected by the sample pulses,
the largest voltage would tend to indicate the peak and
hence the amplitude of the input signal. A sampling is
performed on an analog signal only when a conversion
is required.
Quantization
Quantization takes the sampled analog value and
converts it to the nearest binary value or quantity. The
accuracy of a binary quantity is limited to the value
of the least significant bit (20 ). In the example in figure
13-1, bit 20 was the 1 mile bit, meaning the smallest
value that could be indicated was 1 mile and the greatest
accuracy was plus or minus 1 mile. Smaller values of
1/2 or 1/4 miles or less could not be indicated.
Quantization, in effect, rounds out the conversion to
the value of the least significant bit (LSB).
Encoding
The encoding operation reduces the result of the
conversion to a binary code acceptable to the digital
equipments that use the data. There is a variety of
coding systems in use.
You have already been
introduced to one of the most common ones, natural
binary code. This binary code expresses quantities as
a weighted sum.
Each bit position represents a
specified value when set. The sum of the values of the
set bits defines the value of the quantity. The bit with
Figure 13-2.—Sampling pulses.
13-3
