handbook of modern sensors

5.4 Analog-to-Digital Converters 185
Fig. 5.30. Resolution enhancement circuit.
To make the conversion valid, the input signal V in must not change until all of the
bits are tried; otherwise, the digital reading may be erroneous. To avoid any problems
with the changing input, a successive-approximation converter is usually supplied
with a sample-and-hold (S&H) circuit. This circuit is a short-time analog memory
which samples the input signal and stores it as a dc voltage during an entire conversion
cycle.
5.4.5 Resolution Extension
In a typical data acquisition system, a monolithic microcontroller often contains an
analog-to-digital converter, whose maximum resolution is often limited to 8 bits and
sometimes to 10 bits. When the resolution is higher, 12 or even 14 bits, either the
cost becomes prohibitively high or the on-the-chipA/D converter may possess several
undesirable characteristics. In most applications, 8 or 10 bits may not be nearly enough
for the correct representation of a stimulus. One method of achieving higher resolution
is to use a dual-slope A/D converter whose resolution is limited only by the available
counter rate and the speed response of a comparator. 4 Another method is to use an
eight-bit A/D converter (e.g., of a successive-approximation type) with a resolution
extension circuit. Such a circuit can boost the resolution by several bits, (e.g., from 8 to
12). A block diagram of the circuit is shown in Fig.5.30. In addition to a conventional
eight-bit A/D converter, it includes a D/A converter, a subtraction circuit, and an
amplifier having gain A. In the ASIC or discrete circuits, a D/A converter may be
shared with an A/D part (see Fig. 5.29A).
The input signal V in has a full-scale value E; thus for an eight-bit converter, the
initial resolution will be
R 0 =
E
2 8 − 1 = E
255 , (5.35)
which is expressed in volts per bit. For instance, for a 5-V full scale, the eight-bit
resolution is 19.6 mV/bit. Initially, the multiplexer (MUX) connects the input signal
to the A/D converter, which produces the output digital value, M (expressed in bits).
Then, the microprocessor outputs that value to a D/A converter, which produces
4 A resolution should not be confused with accuracy.