Teledyne 6600 - Manual (Page 107)

Part III: 1-17
Oil in Water Part III
Teledyne Analytical Instruments
The chart speed must be at least 1 inch per hour.
A stripchart recorder is recommended.
3.7 The Process Alarm System
Refers to the control unit part of this manual for interconnection and
programming.
3.8 The amplifier PCB
This board, C67999, contains a differential amplifier. It will take the
difference between the Measurement and the Reference signals to create the
actual output of the amplifier. There are gain settings that are under the
control of the micro controller in the second and the third stage before
delivering the signal to the ADC on the Main PCB
.
3.8.1 Auto Zero Circuit
When the oil-in-water sampling system is in long term operation, a zero
signal drift may be caused by various conditions, among which are physical
changes in sample cell conditions (deposits on sample cell windows, etc.),
source and detector changes and electronics drift (with temperature change,
for example).
Periodic compensation for zero drift is accomplished by electronically
nulling the zero offset, with an equal but opposite signal, while zero fluid is
flowing through the system. Thus, a zero point is obtained, and subsequent
sample measurements will produce a signal representing the difference
between the sample measurement signal and the zero reference signal.
The differential amplifier U4 is to be zeroed by signals fed from the
multiple channel DAC on the Main PCB. There are two signals: A Coarse,
and a Fine adjustments. The coarse adjustment is fed thru J2-4 and the Fine
adjustment is fed thru J2-3. Both signals can swing between 0 to 5 volts. But
they have a different effect on the output of the amplifier due the series
resistor value size of each one. The simplified schematic of the amplifier is
shown below. As it can be seen under ideal conditions, setting both the