Page 245 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes
12.8.3 Section Counters
Another method for reducing the total conversion time of a simple counter converter is to divide
the counter into sections. Such a configuration is called a section counter. To determine how
the total conversion time might be reduced by this method, assume that we have a standard
8-bit counter. If this counter is divided into two equal counters of 4 bits each, we have a section
converter. The converter operates by setting the section containing the four LSBs to all 1s and then
advancing the other sections until the ladder voltage exceeds the input voltage. At this point the
four LSBs are all reset, and this section of the counter is then advanced until the ladder voltage
equals the input voltage.
4
Notice that a maximum of 2 = 16 counts is required for each section to count full scale. Thus this
4
5
method requires only 2 * 2 = 2 = 32 counts to reach full scale. This is a considerable reduction
over the 2 = 256 counts required for the straight 8-bit counter. There is, of course, some extra time
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required to set the counters initially and to switch from counter to counter during the conversion.
This logical operation time is very small, however, compared with the total time saved by this
method.
This type of converter is quite often used for digital voltmeters, since it is very convenient to
divide the counters by counts of 10. Each counter is then used to represent one of the digits of
the decimal number appearing at the output of the voltmeter.

An ADC can resolve a signal to only a certain number of bits of resolution,
called the effective number of bits (ENOB).

12.9 Dual-Slope A/D Conversion

Up to this point, our interest in different methods of A/D conversion has centred on reducing
the actual conversion time. If a very short conversion time is not a requirement, there are
other methods of A/D conversion that are simpler to implement and much more economical.
Basically, these techniques involve comparison of the unknown input voltage with a reference
voltage that begins at zero and increases linearly with time. The time required for the reference
voltage to increase to the value of the unknown voltage is directly proportional to the
magnitude of the unknown voltage, and this time period is measured with a digital counter.
This is referred to as a single-ramp method, since the reference voltage is sloped like a ramp.
A variation on this method involves using an operational amplifier integrating circuit in a
dual-ramp configuration. The dual-ramp method is very popular, and widely used in digital
voltmeters and digital panel meters. It offers good accuracy, good linearity, and very good
noise-rejection characteristics.

12.9.1 Single-Ramp A/D Converter

Let us take a look at the single-ramp A/D converter in Figure 12.32. The heart of this converter
is the ramp generator. This is a circuit that produces an output voltage ramp as shown in Figure
12.33a. The output voltage begins at zero and increases linearly up to a maximum voltage Vm.
It is important that this voltage be a straight line-that is, it must have a constant slope. For
instance, if V = 1.0 Vdc, and it takes 1.0 ms for the ramp to move from 0.0 up to 1.0 V, the
m
slope is 1 V/ms, or 1000 V/s.

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