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An n-bit simultaneous ADC requires 2n comparators
and this is a huge disadvantage when high resolution is needed. This can be
resolved by using a counter type ADC, where a single comparator with variable
reference voltage suffices for any desired accuracy. This type of comparator is shown in the
following schematic diagram.
This circuit works as follows. Initially, the counter is
reset to all 0s. When a convert signal appears on the start line, the input
gate is enabled and the clock pulses are applied to the clock input of the
counter. The counter advances through its normal binary count sequence. The
counter output feeds a D/A converter and the staircase waveform generated at
the output of the D/A converter forms one of the inputs of the comparator. The
other input to the comparator is the analogue input signal. Whenever the D/A
converter’s output exceeds the analog input voltage, the comparator changes
state. The gate is disabled and the counter stops. The counter output at that
instant of time is then the required digital output corresponding to the analog
input signal.
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 higher-resolution A/D converters. The drawback with
this converter is that the required conversion time is longer. Since the counter
always begins from the all 0s position and counts through its normal binary
sequence, it may require as many as 2n counts before conversion is
complete. Average conversion time of a counter method ADC is 2n-1 clock cycles’ time.
For an example, If there is a four-bit converter and a 1 MHz
clock, the average conversion time would be 2n-1 =8 micro seconds. It would be as large as 0.5 ms for a
10-bit converter of this type at a 1 MHz clock rate. In fact, the conversion
time doubles for each bit added to the converter. Thus, the resolution can be
improved only at the cost of a longer conversion time. This makes the
counter-type A/D converter unsuitable for digitizing rapidly changing analog
signals.