478 Chapter 11 Computer peripherals
Figure 11.61 The sample
and hold circuit.
Before we describe ADCs in detail, we look
at the sample and hold circuit used to freeze
time-varying analog signals prior to their
conversion. This circuit is sometimes called a
follow and hold circuit. We mentioned this
circuit when we discussed aperture time.
Analog input R vin
√WV
The diode bridge
is a switch that can be
opened or gnd closed
electronically.
(a) Equivalent circuit.
>VoAnalog o
output
The sample and hold circuit
Like many other analog circuits, the sample and
hold (S/H) circuit is simple in principle but very
complex in practice. The divergence
between theory and practice stems from the
effect of second-or even third-order nonThe output is a
linearities of analog circuits. Such problems
copy of the
input.
nalo o
don’t affect digital circuits.
→Vo A g
output
Figure 11.61 gives the circuit of a sample and
hold amplifier. Readers without a background in
gnd electronics may skip the details of this circuit’s
(b) Equivalent circuit when switch closed.
operation—all it does is to charge a capacitor to
the same level as the
gnd
relatively constant input.
The capacitor
holds the output
If we forget the diode bridge and regard the
(c) Equivalent circuit when switch open.
constant.
input resistor, R, as being directly connected to
→Vo Analog o
the inverting terminal of the operational
Figure 11.62 Operation of the sample and
output
amplifier, we have a simple inverting buffer with
hold circuit.
unity gain (see Fig. 11.62(a)). That is, Vout = — Vin.
Assume also that the capacitor C has negligible
effect on the circuit.
type of A/D converter, analog-to-digital
The diode bridge in Fig. 11.61 acts as an on/off switch that
conversion is performed in a roundabout way. In this section, we
describe three types of A/D converter: the parallel converter (the either connects the analog input to the inverting terminal of the
only direct A/D converter), the feedback converter, and the inte- op-amp via R, or isolates the inverting terminal from the input.
When the switch is in the closed position, the S/H
grating converter.
Analog
input R
input signal, and then connect the capacitor to its output
terminals. For a short time, the voltage on the capacitor remains
constant, allowing the ADC to perform a conversion with a