Welch Allyn Model 767
NEC UPD78064 uP
16 bit
Timer
Horn
Driver
CPU
MODEL 767 PCA
A/D
Converter
Circuitry
Voltage
Reference
Circuitry
LCD
Driver
Serial
I/O
Serial Port
Probe
Warmer
Circuitry
Liquid
Crystal
Display
Mode Button
PROBE ASSEMBLY
Probe
Warmer
Thermistor
Four
Conductor
Coil Cord
Probe Connector
M767 WALL
TRANSFORMER
Probe
Logic
Circuitry
Horn
Backlight
Circuit
AC TO DC
CONVERTER
CIRCUITRY
Test Port J3
J7J8
FIGURE 3- SYSTEM BLOCK DIAGRAM
Temperature Measurement A/D Converter
The primary function of the temperature measurement A/D converter circuit is to convert a measured
resistance into a 16-bit word, which is then used by the microprocessor to calibrate itself and calculate
a temperature.
Theory of Operation
The A/D circuit is made up of 4 major parts:
· The resistance to be measured (either the probe thermistor, the PTBCAL resistor, or the HICAL
resistor).
· The timing capacitor (C21, 0.33uF) and associated driver circuitry
(Q1-Q4, Q9, R15, and R30).
· A low bias dual voltage comparator (U2, TLC555).
· A 16 bit timer internal to the microprocessor.
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