tmp03fruz-reel7 Analog Devices, Inc., tmp03fruz-reel7 Datasheet - Page 10

• Manufacturer Part Number: tmp03fruz-reel7
• Description: Serial Digital Output Thermometers
• Manufacturer: Analog Devices, Inc.

TMP03/TMP04
APPLICATIONS INFORMATION
Supply Bypassing
Precision analog products, such as the TMP03, require a well-
filtered power source. Since the TMP03 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 induc-
tance.
If possible, the TMP03 should be powered directly from the
system power supply. This arrangement, shown in Figure 3, will
isolate the analog section from the logic switching transients. Even
if a separate power supply trace is not available, however, gener-
ous supply bypassing will reduce supply-line induced errors.
Local supply bypassing consisting of a 10 µF tantalum electro-
lytic in parallel with a 0.1 µF ceramic capacitor is recommended
(Figure 4a).
The quiescent power supply current requirement of the TMP03
is typically only 900 µA. 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 4b).
10 F
0.1 F
TTL/CMOS
CIRCUITS
LOGIC
POWER SUPPLY
TMP03/
TMP04
5V
GND
5V
V+
D
OUT
+
10 F
TANT
10 F
0.1 F
0.1 F
TMP03/
TMP04
TMP03/
TMP04
GND
V+
5V
50
D
OUT
TMP03 Output Configurations
The TMP03 (Figure 5a) 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
pulsewidth and introduce measurement errors. The NPN tran-
sistor has a breakdown voltage of 18 V.
The TMP04 has a “totem-pole” CMOS output (Figure 5b) 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 ca-
pacitance is large, for example when driving a long cable, an
external buffer may improve accuracy. See the “Remote Tem-
perature 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 6, 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 com-
mon-mode voltages exist, an opto-isolator can be used to isolate
the output (Figure 7a). 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 7b). An opto-
isolator with an integral logic gate output, such as the H11L1
from Quality Technology, can also be used (Figure 8).
TMP03
TMP03
GND
5V
V+
D
OUT
D
OUT
3.3V
3.3k
TMP04
TO LOW VOLTAGE
LOGIC GATE INPUT
V+
D
OUT