TMP04 AD [Analog Devices], TMP04 Datasheet - Page 10

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

TMP03/TMP04
APPLICATIONS INFORMATION
Supply Bypassing
Precision analog products, such as the TMP03/TMP04, require
a well filtered power source. Since the TMP03/TMP04 operate
from a single +5 V supply, it seems convenient to simply tap
into the digital logic power supply. Unfortunately, the logic
supply is often a switch-mode design, which generates noise in
the 20 kHz to 1 MHz range. In addition, fast logic gates can
generate glitches hundred of millivolts in amplitude due to
wiring resistance and inductance.
If possible, the TMP03/TMP04 should be powered directly
from the system power supply. This arrangement, shown in
Figure 27, will isolate the analog section from the logic switching
transients. Even if a separate power supply trace is not available,
however, generous supply bypassing will reduce supply-line
induced errors. Local supply bypassing consisting of a 10 F
tantalum electrolytic in parallel with a 0.1 F ceramic capacitor
is recommended (Figure 28a).
Figure 27. Use Separate Traces to Reduce Power Supply
Noise
The quiescent power supply current requirement of the
TMP03/TMP04 is typically only 900 A. The supply current
will not change appreciably when driving a light load (such as a
CMOS gate), so a simple RC filter can be added to further
reduce power supply noise (Figure 28b).
Figure 28. Recommended Supply Bypassing for the
TMP03/TMP04
10µF
0.1µF
TTL/CMOS
CIRCUITS
LOGIC
a.
POWER SUPPLY
TMP03/
TMP04
+5V
GND
+5V
V+
D
OUT
10µF
TANT
10µF
0.1µF
0.1µF
b.
TMP03/
TMP04
TMP03/
TMP04
GND
+5V
V+
50
D
OUT
–10–
TMP03/TMP04 Output Configurations
The TMP03 (Figure 29a) has an open-collector NPN output
which is suitable for driving a high current load, such as an
opto-isolator. Since the output source current is set by the pull-
up resistor, output capacitance should be minimized in TMP03
applications. Otherwise, unequal rise and fall times will skew the
pulse width and introduce measurement errors. The NPN
transistor has a breakdown voltage of 18 V.
The TMP04 has a “totem-pole” CMOS output (Figure 29b)
and provides rail-to-rail output drive for logic interfaces. The
rise and fall times of the TMP04 output are closely matched, so
that errors caused by capacitive loading are minimized. If load
capacitance is large, for example when driving a long cable, an
external buffer may improve accuracy. See the “Remote
Temperature Measurement” section of this data sheet for
suggestions.
Interfacing the TMP03 to Low Voltage Logic
The TMP03’s open-collector output is ideal for driving logic
gates that operate from low supply voltages, such as 3.3 V. As
shown in Figure 30, a pull-up resistor is connected from the low
voltage logic supply (2.9 V, 3 V, etc.) to the TMP03 output.
Current through the pull-up resistor should be limited to about
1 mA, which will maintain an output LOW logic level of
<200 mV.
Remote Temperature Measurement
When measuring a temperature in situations where high
common-mode voltages exist, an opto-isolator can be used to
isolate the output (Figure 31a). The TMP03 is recommended in
this application because its open-collector NPN transistor has a
higher current sink capability than the CMOS output of the
TMP04. To maintain the integrity of the measurement, the
opto-isolator must have relatively equal turn-on and turn-off
times. Some Darlington opto-isolators, such as the 4N32, have
a turn-off time that is much longer than their turn-on time. In
this case, the T1 time will be longer than T2, and an erroneous
reading will result. A PNP transistor can be used to provide
greater current drive to the opto-isolator (Figure 31b). An opto-
isolator with an integral logic gate output, such as the H11L1
from Quality Technology, can also be used (Figure 32).
Figure 29. TMP03/TMP04 Digital Output Structure
Figure 30. Interfacing to Low Voltage Logic
TMP03
TMP03
a.
GND
+5V
V+
D
OUT
D
OUT
+3.3V
3.3k
TMP04
TO LOW VOLTAGE
LOGIC GATE INPUT
b.
V+
D
OUT
REV. 0