handbook of modern sensors

5.4 Analog-to-Digital Converters 181
A major advantage of the integrating-type converters, such as a charge-balanced
V/F converter, is the ability to reject large amounts of additive noise; by integrating the
measurement, noise is reduced or even totally eliminated. Pulses from the converter
are accumulated for a gated period T in a counter. Then, the counter behaves like a
filter having a transfer function in the form
H(f)=
sin πf T
πf T , (5.30)
where f is the frequency of pulses. For low frequencies, the value of this transfer
function H(f) is close to unity, meaning that the converter and the counter make
correct measurements. However, for a frequency 1/T, the transfer function H(1/T)
is zero, meaning that these frequencies are completely rejected. For example, if the
gating time T = 16.67 ms which corresponds to a frequency of 60 Hz (the power line
frequency which is a source of substantial noise in many sensors), then the 60 Hz
noise will be rejected. Moreover, the multiple frequencies (120 Hz, 180 Hz, 240 Hz,
and so on) will also be rejected.
5.4.3 Dual-Slope Converter
Adual-slope converter is very popular; it is used nearly universally in handheld digital
voltmeters and other portable instruments where a fast conversion is not required.
This type of converter performs an indirect conversion of the input voltage. First, it
converts V in into a function of time; then, the time function is converted into a digital
number by a pulse counter. Dual-slope converters are quite slow; however, for stimuli
which do not exhibit fast changes, they are often the converters of choice, due to their
simplicity, cost-effectiveness, noise immunity, and potentially high resolution. The
operating principle of the converter is as follows (Fig. 5.27). Like in a charge-balance
converter, there is an integrator and a threshold comparator. The threshold level is
set at zero (ground) or any other suitable constant voltage. The integrator can be
selectively connected through the analog selector S 1 either to the input voltage or to
the reference voltage. In this simplified schematic, the input voltage is negative, and
the reference voltage is positive. However, by shifting the dc level of the input signal
(with the help of an additional OPAM), the circuit will be able to convert bipolar input
signals as well. The output of the comparator sends a signal to the control logic when
the integrator’s output voltage crosses zero. The logic controls both the selector S 1
and the reset switch S 2 , which serves for discharging the integrating capacitor, C in .
When the start input is enabled, S 1 connects the integrator to the input signal
and the logic starts a timer. The timer is preset for a fixed time interval T . During
that time, the integrator generates a positive-going ramp (Fig. 5.28), which changes
according to the input signal. It should be noted that the input signal does not have to
be constant. Any variations in the signal are averaged during the integration process.
Upon elapsing time T , the integrator output voltage reaches the level
T
V m = V in , (5.31)
R in C in