Welch Allyn SureTemp 670 Especificaciones descargar pdf (Pagina 28)

Technical Manual

4-6 Welch-Allyn, Inc.

Q4 and Q5 and the base resistors R15 and R30 form the level shifter and drive circuit

for the fixed end of the timing capacitor. R15 (4.7K) is selected to give adequate

saturation on-resistance. R30 (10K) is selected so that a float on the input will not

cause damage from cross-conduction if the input is left floating, yet supply enough

current to the timing capacitor during recovery. Q4 and Q9 exhibit only a few millivolts

of saturation voltage, which does not affect the accuracy of the A/D as described

above.

•

C21 is the surface mount timing capacitor. This capacitor is selected for low

dielectric absorption, hence the high voltage rating (50V).

•

D4 and R16 protect the comparator from input undervoltage when the timing

capacitor (C21) is discharged. D4 also reduces the recovery time by limiting the

voltage at the beginning of recovery to 0.6 volts instead of 2/3VREF.

•

C5 provides the first line of defense from EMI (Electro Magnetic Interference)

coming in on the probe. R33 and C18 protect Q1 from rectifying EMI in the

substrate diode. R16 protects the comparator input from overcurrent.

Probe Identification Logic

Basic Function

The probe logic circuitry is needed to determine which type of probe is connected to

the device. This allows the microprocessor to initiate the proper algorithm for

converting a thermistor measurement into a temperature. Figure 4 lists the probe

logic circuitry and Figure 5 shows the logic diagram.

PROBE LOGIC

SIGNAL ORAL RECTAL CAL KEY NO PROBE

PROBE_0 0 0 1 1

PROBE_1 0 1 0 1

Figure 4 - Probe Logic Circuitry

PROBE_1

PROBE_0

VCC

R10

R11

C19

0.01UF

C20

0.01UF

J1F

J1C

C U T H E R E

F O R R E C T A L

C U T H E R E

F O R C A L K E Y

Probe Connector

PCB

To Microprocessor

NO CUT FOR ORAL

Figure 5 - Probe Logic Diagram

1 2 ... 23 24 25 26 27 28 29 30 31 32 33 ... 48 49

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Welch Allyn SureTemp 670 Especificaciones Pagina 28

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