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Digital Circuits and Logic Design

Notes 12.6 A/D Converter-Counter Method

A higher-resolution A/D converter using only one comparator could be constructed if a variable
reference voltage were available. This reference voltage could then be applied to the comparator,
and when it became equal to the input analog voltage, the conversion would be complete.

To construct such a converter, let us begin with a simple binary counter. The digital output signals
will be taken from this counter, and thus we want it to be an n-bit counter, where n is the desired
number of bits. Now let us connect the output of this counter to a standard binary ladder to form
a simple D/A converter. If a clock is now applied to the input of the counter, the output of the
binary ladder is the familiar staircase waveform shown in Figure 12.16. This waveform is exactly
the reference voltage signal we would like to have for the comparator! With a minimum of gating
and control circuitry, this simple D/A converter can be changed into the desired A/D converter.
Figure 12.23 shows the block diagram for a counter-type A/D converter. The operation of the
counter is as follows. First, the counter is reset to all 0s. Then, when a convert signal appears
on the START line, the gate opens and clock pulses are allowed to pass through to the input of
the counter. The counter advances through its normal binary count sequence, and the staircase
waveform is generated at the output of the ladder.

Figure 12.23: Counter-type A/D Converter

START

Clock Gate and Counter
control
N lines
Level amplifiers

Comp. N lines
Ref.
Analog voltage
input Binary ladder
voltage
N lines

Digital output
This wave form is applied to one side of the comparator, and the analog input voltage is applied
to the other side. When the reference voltage equals (or exceeds) the input analog voltage, the gate
is closed, the counter stops, and the conversion is complete. The number stored in the counter is
now the digital equivalent of the analog input voltage.
Notice that this converter is composed of a D/A converter (the counter, level amplifiers, and
the binary ladder), one comparator, a clock, and the gate and control circuitry. This can really
be considered as a closed-loop control system. An error signal is generated at the output of the
comparator by taking the difference between the analog input signal and the feedback signal
(staircase reference voltage). The error is detected by the control circuit, and the clock is allowed
to advance the counter. The counter advances in such a way as to reduce the error signal by
increasing the feedback voltage. When the error is reduced to zero, the feedback voltage is equal
to the analog input signal, the control circuitry stops the clock from advancing the counter, and
the system comes to rest.
The counter-type A/D converter provides a very good method for digitizing to a high resolution.
This method is much simpler than the simultaneous method for high resolution, but the conversion

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