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handbook of modern sensors

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14.6 Thermal Detectors 433<br />

Fig. 14.23. Output signals <strong>of</strong> the voltage follower<br />

(A) and current-to-voltage converter (B) in response<br />

to a step function <strong>of</strong> a thermal radiation.<br />

(A)<br />

(B)<br />

where R 1 and R 0.1 are the resistances measured respectively at 1 and 0.1 V. Usually,<br />

the VCR is negative; that is, the resistance value drops with an increase in voltage<br />

across the resistor (Fig. 14.24A). Since the pyroelectric sensor’s output is proportional<br />

to the product <strong>of</strong> the pyroelectric current and the bias resistor, the VCR results<br />

in the nonlinearity <strong>of</strong> an overall transfer function <strong>of</strong> the sensor plus circuit. A highimpedance<br />

resistor is fabricated by depositing a thin layer <strong>of</strong> a semiconductive ink<br />

on a ceramic (alumina) substrate, firing it in a furnace and subsequently covering<br />

the surface with a protective coating. A high-quality, relatively thick (at least 50 µm<br />

thick) hydrophobic coating is very important for protection against moisture, because<br />

even a small amount <strong>of</strong> water molecules may cause oxidation <strong>of</strong> the semiconductive<br />

layer. This causes a substantial increase in the resistance and poor long-term stability.<br />

A typical design <strong>of</strong> a high-impedance resistor is shown in Fig. 14.24B.<br />

(A)<br />

(B)<br />

Fig. 14.24. High-impedance resistor: (A) VCRs for three different types <strong>of</strong> the resistor;<br />

(B) structure <strong>of</strong> a resistor on an alumina substrate.

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