Teledyne 6600 3.7The Process Alarm System, 3.8The amplifier PCB, 3.8.1Auto Zero Circuit, Part III

Oil in Water Part III

The chart speed must be at least 1 inch per hour.

A stripchart recorder is recommended.

3.7The Process Alarm System

Refers to the control unit part of this manual for interconnection and programming.

3.8The 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.

3.8.1Auto 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

Teledyne Analytical Instruments Part III: 1-17