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TMP03F

Serial Digital Output Thermometers

Analog Devices 

TMP03/TMP04

5V

620⍀

V+

TMP03

DOUT

GND

VLOGIC

OPTO-COUPLER 4.7k⍀

a.

5V

V+

TMP03

10k⍀

2N2907

VLOGIC

OPTO-COUPLER

270⍀

430⍀

4.3k⍀

DOUT

GND

b.

Figure 7. Optically Isolating the Digital Output

5V

5V

680⍀

V+

TMP03

DOUT

GND

H11L1

4.7k⍀

Figure 8. An Opto-Isolator with Schmitt Trigger Logic

Gate Improves Output Rise and Fall Times

The TMP03 and TMP04 are superior to analog-output trans-

ducers for measuring temperature at remote locations, because

the digital output provides better noise immunity than an analog

signal. When measuring temperature at a remote location, the

ratio of the output pulses must be maintained. To maintain the

integrity of the pulsewidth, an external buffer can be added. For

example, adding a differential line driver such as the ADM485

permits precise temperature measurements at distances up to

4000 ft. (Figure 9). The ADM485 driver and receiver skew is

only 5 ns maximum, so the TMP04 duty cycle is not degraded.

Up to 32 ADM485s can be multiplexed onto one line by pro-

viding additional decoding.

As previously mentioned, the digital output of the TMP03

provides excellent noise immunity in remote measurement appli-

cations. The user should be aware, however, that heat from an

external cable can be conducted back to the TMP03. This heat

conduction through the connecting wires can influence the

temperature of the TMP03. If large temperature differences

exist within the sensor environment, an opto-isolator, level

shifter or other thermal barrier can be used to minimize measure-

ment errors.

5V

V+

DOUT

TMP04

GND

DI

NC

5V

DE

VCC

B

A

ADM485

Figure 9. A Differential Line Driver for Remote Tempera-

ture Measurement

Microcomputer Interfaces

The TMP03 output is easily decoded with a microcomputer.

The microcomputer simply measures the T1 and T2 periods in

software or hardware, and then calculates the temperature using

the equation in the Output Encoding section of this data sheet.

Since the TMP03’s output is ratiometric, precise control of the

counting frequency is not required. The only timing require-

ments are that the clock frequency be high enough to provide

the required measurement resolution (see the Output Encoding

section for details) and that the clock source be stable. The

ratiometric output of the TMP03 is an advantage because the

microcomputer’s crystal clock frequency is often dictated by the

serial baud rate or other timing considerations.

Pulsewidth timing is usually done with the microcomputer’s

on-chip timer. A typical example, using the 80C51, is shown in

Figure 10. This circuit requires only one input pin on the micro-

computer, which highlights the efficiency of the TMP04’s

pulsewidth output format. Traditional serial input protocols,

with data line, clock and chip select, usually require three or

more I/O pins.

5V

V+

DOUT

TMP04

GND

INPUT

PORT 1.0

OSC

TIMER 0

(16-BITS)

80C51

MICROCOMPUTER

TIMER 1

(16-BITS)

،12 TMOD REGISTER

TIMER 0 TIMER 1

TCON REGISTER

TIMER 0 TIMER 1

Figure 10. A TMP04 Interface to the 80C51 Microcomputer

The 80C51 has two 16-bit timers. The clock source for the timers

is the crystal oscillator frequency divided by 12. Thus, a crystal

frequency of 12 MHz or greater will provide resolution of 1 µs

or less.

The 80C51 timers are controlled by two dedicated registers. The

TMOD register controls the timer mode of operation, while

TCON controls the start and stop times. Both the TMOD and

TCON registers must be set to start the timer.

REV. B

–11–

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