Guildline Autosal 8400B - 3.6. DISPLAY CIRCUITRY

To Next Page

To Previous Page

Section 3

TM8400B-L-00

1 November 2004

3-6

The following references are to Figure 3.2. The precision reference generates precise stable +1

and -1 volt references. The chopper (U209) is driven by the clock signals to alternate the two

reference values and provide a low frequency square wave ±1 volt signal. U208A buffers this

reference, and drives the cell voltage comparator circuit and the suppression resistor chain. The

voltage across the cell resistance is monitored by buffers U200 and U203. These signals are

summed with reference signal (U208A), and a DC bias signal from U208C and fed to U208B

which provides the cell current to keep the voltage between P1 and P2 constant and equal to the

reference voltage. U208D keeps P2 at ground potential - sinking or sourcing current as

necessary. Keeping P2 at ground potential prevents a leakage current to the heat exchanger

which is also at ground potential. In the cell current path, Rcell is the conductivity cell, Rs is the

standardization resistor Rtemp is the temperature compensation resistor chain, Rmatching is the

precisely cut cell matching resistor. The voltage across the matching resistor and the selected

temperature compensating resistance is buffered by U205A and U205D. U205B is an inverting

gain stage. The voltage level from the suppression chain is buffered by U205C, and the three

signals are summed by U207 to provide a signal proportional to the conductivity of the cell. The

dechopper (U212) rectifies and smooths the AC signal to provide a DC signal, and U206

provides a final gain and offset compensation stage. The same precision reference is used for the

reference of the analog to digital convertor.

3.6. DISPLAY CIRCUITRY

The output from the conductivity measuring circuit is changed to a binary coded decimal number

by the meter circuit (16504.01.04). This number is displayed on a 4 digit LED (light emitting

diode) readout and is available for electronic data logging through the rear panel BCD connector.

The analog to decimal conversion is performed by a commercial 4½ digit analog to digital

integrated circuit system.