TMP04 AD [Analog Devices], TMP04 Datasheet - Page 11

• Manufacturer Part Number: TMP04
• Description: Serial Digital Output Thermometers
• Manufacturer: AD [Analog Devices]

REV. 0
Figure 32. An Opto-Isolator with Schmitt Trigger Logic
Gate Improves Output Rise and Fall Times
The TMP03 and TMP04 are superior to analog-output
transducers 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 pulse width, 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 33). 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 providing additional decoding.
As previously mentioned, the digital output of the TMP03/
TMP04 provides excellent noise immunity in remote measurement
applications. The user should be aware, however, that heat from
an external cable can be conducted back to the TMP03/TMP04.
This heat conduction through the connecting wires can influence
the temperature of the TMP03/TMP04. If large temperature
differences exist within the sensor environment, an opto-
isolator, level shifter or other thermal barrier can be used to
minimize measurement errors.
Figure 31. Optically Isolating the Digital Output
TMP03
TMP03
TMP03
GND
GND
GND
+5V
+5V
+5V
V+
V+
V+
D
D
D
OUT
OUT
OUT
4.3k
10k
680
620
2N2907
270
a.
b.
H11L1
OPTO-COUPLER
OPTO-COUPLER
+5V
V
V
LOGIC
LOGIC
430
4.7k
4.7k
–11–
Figure 33. A Differential Line Driver for Remote Tempera-
ture Measurement
Microcomputer Interfaces
The TMP03/TMP04 output is easily decoded with a micro-
computer. The microcomputer simply measures the T1 and T2
periods in software or hardware, and then calculates the temp-
erature using the equation in the Output Encoding section of
this data sheet (page 4). Since the TMP03/TMP04’s output is
ratiometric, precise control of the counting frequency is not
required. The only timing requirements 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/TMP04 is an advantage because the microcomputer’s
crystal clock frequency is often dictated by the serial baud rate
or other timing considerations.
Pulse width timing is usually done with the microcomputer’s
on-chip timer. A typical example, using the 80C51, is shown in
Figure 34. This circuit requires only one input pin on the
microcomputer, which highlights the efficiency of the TMP04’s
pulse width output format. Traditional serial input protocols,
with data line, clock and chip select, usually require three or
more I/O pins.
Figure 34. 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 s 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.
TMP04
GND
+5V
V+
D
OUT
TMP04
+5V
GND
V+
2
3
D
OUT
INPUT
PORT 1.0
MICROCOMPUTER
1
(16 BITS)
TIMER 0
(16 BITS)
TIMER 1
80C51
+5V
NC
OSC
4
1
2
DI
3
TMP03/TMP04
DE
5
12
ADM485
V
8
CC
B
A
TIMER 0
TIMER 0
TMOD REGISTER
TCON REGISTER
7
6
TIMER 1
TIMER 1