Teledyne 7120 - NDIR analyzer - Manual (Page 28)

Photometric Analyzer Start-up and Operation
3-5 Teledyne Analytical Instruments
3.4 Circuit Description
Infrared energy light pulses are focused on the lead Selenide (PbSe) sensor at
a 400 Hz rate; this causes its photoconduction to change in proportion to the
IR signal strength. Therefore photoconduction occurs, there is a minute
change in the voltage level across the sensor which is noted by the two
voltage followers, UlA and UlB, located on preamplifier PCB C72758, refer
to schematic C28165. The infrared energy pulsations are converted into a
small 400 Hz electronic waveform.
The is signal is amplified by the differential amplifier, UlC, whose capacitor-
coupled input blocks the DC voltage present in the input signal. This
differential amplifier is designed to have a very high common mode rejection
ratio. This output signal will be a series of pulses, the “reference” and
“measurement” signals one after the other. These are further amplified by
using the booster amplifier U1D.
The reference and measurement signals are identified by an optical system
that detects the position of the chopper wheel.
The light from a light source, the optical coupler lamp, is reflected by the
white parts of the collar on the chopper wheel. m is reflected light causes a
photo conductive element to conduct. U5A and U5B sense this and generate
a square wave of corresponding width. These ID pulse trains are utilized to
separate the measuring and reference signals.
The source output can fluctuate due to line voltage fluctuations, aging of the
source, or contamination of the cells, resulting in corresponding signal level
changes which would cause an error in the reading. This is corrected by the
automatic level control (ALC) circuit, V3. Both the reference and
measurement pulses are passed through the ALC, but only the reference
pulse is used to control the gain of the ALC. As a result, the reference pulse,
at its output, remains at a constant level despite any changes in the energy
reaching the detector.
The complete video signal is first clamped to the zero level by zero restorer
circuit U2C and U2D and the reference signal is separated from the video
signal by ~r switch Q1, which is controlled by the ID pulses from U5A. This
signal is fed to the rectifier input of t]he ALC as a controlling signal for the
gain control circuit. m e output reference signal level is always constant and
is determined by the internal resistors of the amplifier. m e measurement
signal level will be equal to the input signal multiplied by the gain of the