handbook of modern sensors

162 5 Interface Electronic Circuits
(A)
(B)
Fig. 5.10. Charge-to-voltage (A) and current-to-voltage (B) converters.
which generate very small charges (on the order of picocoulombs, pC) or currents (on
the order of picoamperes). A basic circuit of a charge-to-voltage converter is shown
in Fig. 5.10A. A capacitor, C, is connected into a feedback network of an OPAM. Its
leakage resistance r must be substantially larger than the impedance of the capacitor
at the lowest operating frequency. A good film capacitor is usually recommended
along with a good quality printed circuit board where the components are coated with
conformal coating.
A transfer function of the converter is
V out =− Q
C . (5.12)
Special hybrid charge-sensitive preamplifiers are available for precision applications.
One example is DN630 from ThermOptics, Inc. (www.thermoptics.com). The amplifier
can operate with sources of less than 1 pF capacitance. An internally connected
1-pF capacitor sets the gain of the amplifier to 1 V/pC sensitivity. The gain can be
reduced by connecting one or a combination of the internal capacitor array to the
input of the amplifier. It features low noise and has less than 5 ns rise and fall times.
Many sensors can be modeled as capacitors. Some capacitive sensors are active;
that is, they require an excitation signal. Examples are microphones, capacitive
force and pressure transducers, and humidity detectors. Other capacitive sensors are
passive; that is, they directly convert a stimulus into an electric charge or current. Examples
are the piezoelectric and pyroelectric detectors. There are also noncapacitive
sensors that can be considered as current generators. An example is a photodiode.
A current-generating sensor is modeled by a leakage resistance, r, connected in
parallel with a current generator that has an infinitely high internal resistance (Fig.
5.11). The sensor generates current, i, which has two ways to outflow: to the sensors
leakage resistance, r, as current, i 0 , and the other, i out , toward the interface circuit
input impedance, Z L . Naturally, current i 0 is useless and to minimize the error of
the current-to-voltage conversion, the leakage resistance of the sensor must be much
larger than the impedance of the interface circuit.
Ohm’s law suggests that to convert electric current i out into voltage, current should
pass through an appropriate impedance and the voltage drop across that impedance