handbook of modern sensors

14.6 Thermal Detectors 431
A dual element is often fabricated from a single flake of a crystalline material (Fig.
14.21B). The metallized pattern on both sides of the flake form two serially connected
capacitors C 1 and C 2 . Figure 14.21C shows an equivalent circuit of a dual-pyroelectric
element. This design has the benefit of a good balance of both elements, thus resulting
in a better rejection of common-mode interferences. Note that the sensing element
exists only between the opposite electrodes and the portion of the flake that is not
covered by the electrodes, is not participating in the generation of a useful signal.
A major problem in the design of pyroelectric detectors is in their sensitivity to
mechanical stress and vibrations. All pyroelectrics are also piezoelectrics; therefore,
although sensitive to thermal radiation, the pyroelectric sensors are susceptible to
interferences which are called “microphonics” sometimes. For better noise rejection,
the crystalline element must be mechanically decoupled from the outside, especially
from the terminals and the metal can.
A pyroelectric element (a crystal flake plus two opposite electrodes) can be modeled
by a capacitor connected in parallel with a leakage resistor. The value of that
resistor is on the order of 10 12 − 10 14 . In practice, the sensor is connected to a
circuit which contains a bias resistor R b and an impedance converter (“circuit” in
Fig. 14.21A). The converter may be either a voltage follower (e.g., JFET transistor)
or a current-to-voltage converter. The voltage follower (Fig. 14.22A) converts the
high output impedance of the sensor (capacitance C in parallel with a bias resistance
R b ) into the output resistance of the follower which, in this example, is determined
by the transistor’s transconductance in parallel with 47 k. The advantage of this
circuit is in its simplicity, low cost, and low noise. A single JFET follower is the most
cost-effective and simple; however, it suffers from two major drawbacks. The first is
the dependence of its speed response on the so-called electrical time constant, which
is a product of the sensor’s capacitance C and the bias resistor R b :
τ e = CR b . (14.21)
For example, a typical dual sensor may have C = 40 pF and R b = 50 G, which
yield τ e = 2 s, corresponding to a first-order frequency response with the upper cutoff
frequency at the 3-dB level equal to about 0.08 Hz—a very low frequency indeed.
(A)
(B)
Fig. 14.22. Impedance converters for pyroelectric sensors: (A) voltage follower with JFET;
(B) current-to-voltage converter with operational amplifier.