handbook of modern sensors

178 5 Interface Electronic Circuits
integrator can be built for the applications, where the stimulus needs to be integrated
over a certain time. The counter accumulates pulses over the gated interval rather
than as an average number of pulses per counting cycle.
Another useful feature of a V/F converter is that its pulses can be easily transmitted
through communication lines. The pulsed signal is much less susceptible to a
noisy environment than a high-resolution analog signal. In the ideal case, the output
frequency f out of the converter is proportional to the input voltage V in :
f out
f FS
= V in
V FS
, (5.25)
where f FS and V FS are the full-scale frequency and input voltage, respectively. For
a given linear converter, ratio f FS /V FS = G is constant and is called a conversion
factor; then,
f out = GV in . (5.26)
There are several known types of V/F converters. The most popular of them are the
multivibrator and the charge-balance circuit.
A multivibrator V/F converter employs a free-running square-wave oscillator
where charge–discharge currents of a timing capacitor are controlled by the input
signal (Fig. 5.23). The input voltage V in is amplified by a differential amplifier (e.g.,
an instrumentation amplifier) whose output signal controls two voltage-to-current
converters (transistors U 1 and U 2 ). A precision multivibrator alternatively connects
timing capacitor C to both current converters. The capacitor is charged for a half of
period through transistor U 1 by the current i a . During the second half of the timing
period, it is discharged by the current i b through transistor U 2 . Because currents i a and
i b are controlled by the input signal, the capacitor charging and discharging slopes
vary accordingly, thus changing the oscillating frequency. An apparent advantage of
this circuit is its simplicity and potentially very low power consumption; however,
its ability to reject high-frequency noise in the input signal is not as good as in the
charge-balance architecture.
Fig. 5.23. Multivibrator type of voltage-to-frequency converter.