Circuit Description - Welch Allyn SureTemp 986 Service Manual

A.2.3 Circuit Description
Model 986 Thermometer
The time constant is measured by counting the time it takes for the
voltage to decay from a fixed initial voltage level to a fixed lower
voltage. The ratio of this pulse width and the pulse width
corresponding to a known calibration resistance (R13, 11.55K) is
used to calculate the measured resistance.
Once the resistance is known, the corresponding temperature is
calculated using the thermistor temperature equation. Because the
M986 uses this 'ratio cal' method for measuring the thermistor
resistance, the device is immune to the variability in gain of
external hardware devices.
Initially, the microprocessor simultaneously discharges both sides
of the capacitor. This is accomplished by bringing A/D TRIGGER
high which turns on Q9 pulling one side of the capacitor (C21) to
ground and turns on Q1 and Q3 via PROBE_SEL, and
RATIOCAL_SEL which allows the other side to discharge through
the thermistor, and R13 (11.55K).
Once the capacitor has been fully discharged, the RATIOCAL
resistor is then selected by switching the probe resistor path off.
The microprocessor then forces the A/D TRIGGER signal to go
low, turning on Q4, which lifts one end of the capacitor up to VREF
(1.8V). Because there can be no instantaneous voltage drop
across the capacitor, the other side of the capacitor immediately
goes to 1.8V. This exceeds the threshold voltage (1.2V) of U2
(LMC555), causing the output A/D_OUT to go low.
At this point, the capacitor begins to discharge from 1.8 V to 0V
through the RATIOCAL resistor. The output of the comparator
goes high again when its input reaches the trigger voltage (0.6
volts). This produces a pulse of length equal to the time it takes for
the capacitor to discharge from 1.8V to 0.6V through the
RATIOCAL resistor.
The microprocessor measures this pulse width using a firmware
implemented 16-bit timer and then after repeating the capacitive
discharge cycle, the same A/D conversion is performed using the
probe resistor as the measured resistance. The microprocessor
performs the following calculation that ratios these two pulse
widths to determine the exact resistance of the thermistor:
( PWthermistor (
Rthermistor
PWhical
The microprocessor then uses the following equation to convert
the measured resistance into a temperature:
Temperature_in_Kelvin
Ra
Appendix A. Theory of Operation
.
) Rhical )
1
3
. .
Rb ln( Rt ) Rc ln( Rt )
33