Cirrus Logic AN300 REV1 - Manual (Page 13)

AN300REV1 13
AN300
4. APPLICATION CIRCUITS
4.1 Thermopile Amplifiers
Figures 16 illustrates a basic diagram of a thermopile. The thermopile is a collection of series-connected thermo-
couples designed to detect spectral radiation. The spectrum of light is first filtered by an optical filter to select the
frequency of light that is to be measured. This light passes through the optical filter and strikes an energy absorbent
coating that converts the light into heat. The thermal change due to the absorbed radiation then causes a change
in the output voltage of the series-connected thermocouples. The resultant output voltage is usually very small and
must be amplified. The magnitude of the output signal is affected by the sensor area, the sensor technology, the
filter optics, and the last of all, the intensity of the radiation.
Figure 16. Typical Thermopile Block Diagram
A wide variety of thermopile devices are available. Various thermopile manufacturers offer thermopiles based upon
thin film technology or upon silicon technology. Thin film devices tend to have lower noise and provide higher sen-
sitivity. Silicon devices are less sensitive and have higher noise levels, but provide better temperature stability. Sil-
icon devices are also less expensive.
Figure 17. Typical Thermopile & Junction Layout
Terminal Connections
15 “Hot” (Active Area) Junctions
“Cold” Junctions
Alternating “Arms” of
Antimony & Bismuth
G
ND
V O
UT
Optical Filter
Aperture
Active Area
Images courtesy of Dexter Research Center, Inc.