Teledyne 6020 - Photometric analyzer - Manual (Page 67)

Part II: 1-5
Photometric Analyzer Operational Theory 1
Teledyne Analytical Instruments
1.3 Photometer Amplifier
The photo detector converts the photo energy striking it to electrical
energy. The magnitude of the photo energy pulses that strike the detector is
determined by absorbance by the sample and the properties of the optical
filters.
The detector output, which is a sequence of pulses that directly reflect
the photo energy transmitted by the measuring and reference filter, is a
measure of the concentration of the component of interest in the sample. The
difference in energy between the measuring and reference pulse is related
exponentially to the concentration of the component of interest.
The photo detector current output is amplified by a current to voltage
(I to E) converting amplifier, followed by a second amplifier. The gain of the
amplifier can be adjusted to obtain any desired output level.
To obtain analyzer options that are linearly related to the concentration
of the component of interest, the output of the I to E converting amplifier is fed
to the input of a logarithmic amplifier, which produces a signal that represents
the logarithm of the output signal of the second amplifier. The output of the
logarithmic amplifier is fed to the input of an inverting amplifier, which acts like
a buffer between log amplifier and switch and inverts the input signal for further
processing.
The output of the inverting amplifier is fed to a magnetically activated
SPDT reed switch, synchronized in such a way that all measuring pulses are
collected on one switch contact and all reference pulses on the other.
The pulses pass through diodes that isolate the integrating networks
from each other. The integrators convert the reference and measuring pulse
energy to a DC level representing them. These reference and measuring DC
levels are applied to the subtracting amplifier in the Control Unit. The output
of the subtractor is a DC voltage linearly related to the concentration of the
component of interest.
From the subtractor, the signal progresses to the analog to digital
converter on the motherboard of the Control Unit.
The microcontroller reads the A to D converter and displays the result
on the front panel.