handbook of modern sensors

5.4 Analog-to-Digital Converters 179
Fig. 5.24. Charge-balance V/F converter.
The charge-balance type of converter employs an analog integrator and a voltage
comparator, as shown in Fig. 5.24. This circuit has such advantages as high speed,
high linearity, and good noise rejection. The circuit is available in an integral form
from several manufacturers—for instance, ADVFC32 and AD650 from Analog Devices,
and LM331 from National Semiconductors. The converter operates as follows.
The input voltage V in is applied to an integrator through the input resistor R in . The
integrating capacitor is connected as a negative feedback loop to the operational amplifier
whose output voltage is compared with a small negative threshold of −0.6
V. The integrator generates a saw-tooth voltage (Fig. 5.26), which, at the moment
of comparison with the threshold, results in a transient at the comparator’s output.
That transient enables a one-shot generator which produces a square pulse of a fixed
duration t os . A precision current source generates constant current i, which is alternatively
applied either to the summing node of the integrator or to its output. The
switch S 1 is controlled by the one-shot pulses. When the current source is connected
to the summing node, it delivers a precisely defined packet of charge Q = it os to the
integrating capacitor. The same summing node also receives an input charge through
the resistor R in , thus the net charge is accumulated on the integrating capacitor C in .
When the threshold is reached, the one-shot is triggered and the switch S 1 changes
its state to high, thus initiating a reset period (Fig. 5.25B). During the reset period,
the current source delivers its current to the summing node of the integrator. The
input current charges the integrating capacitor upward. The total voltage between the
threshold value and the end of the deintegration is determined by the duration of a
one-shot pulse:
V = t os
dV
dt
= t os
i − I in
C in
. (5.27)
When the output signal of the one-shot circuit goes low, switch S 1 diverts current i to
the output terminal of an integrator, which has no effect on the state of the integrating
capacitor C in ; that is, the current source sinks a portion of the output current from the
operational amplifier. This time is called the integration period (Figs. 5.25Aand 5.26).
During the integration, the positive input voltage delivers current I in = V in /R in to the