Welch Allyn SureTemp 670 Specifikace stáhnout pdf (Strana 27)

Theory of Operation

SureTemp

Model 678/SureTemp

Model 679 4-5

In effect, this is a single slope converter which measures the time constant of the

measured resistance combined with a fixed capacitance. 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 M678 uses this

‘ratio cal’ method for measuring the thermistor resistance, the device is immune to a

number of gain errors which can be measured and corrected using software.

Circuit Description

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 capacitor C21 to ground. This also turns on Q1, Q2, and Q3 via PROBE_SEL,

PTBCAL_SEL, and HICAL_SEL, which allows the other side to discharge through the

thermistor R8(12.1K) and R13(11.55K).

Once capacitor C21 has been fully discharged, the probe thermistor is then selected

by switching the PTBCAL and the HICAL resistor paths off. The microprocessor then

forces the A/D TRIGGER signal to go low turning on Q4, which lifts one end of

capacitor C21 up to VREF(2.4v).

Because there can be no instantaneous voltage drop across capacitor C21, the other

side of the capacitor immediately goes to 2.4v. This exceeds the threshold

voltage(1.6v) of U2(TLC555), causing the output A/D_OUT to go low. At this point,

the capacitor begins to discharge from 2.4v to 0v through the thermistor resistor.

The output of the comparator goes high again when its input reaches the trigger

voltage(0.8 volts). This produces a pulse of length equal to the time it takes for the

capacitor to discharge from 1.6v to 0.8v through the thermistor.

The microprocessor measures this pulse width using an internal 16-bit timer and then

the same A/D conversion is performed using the HICAL resistor as the measurand

resistance. The microprocessor performs the following calculation that ratios these

two pulse widths to determine the exact resistance of the thermistor:

Rthermistor

()PWthermistor ( )Rhical

PWhical

The microprocessor then uses the following equation to convert the measured

resistance into a temperature:

Temperature_in_Kelvin

Rb ln( )Rt

Rc ( )ln( )Rt

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 48 49

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Welch Allyn SureTemp 670 Specifikace Strana 27