Data Sheet TMP35/TMP36/TMP37
An offset trim network (f
OUT
OFFSET ) is included with this
circuit to set f
OUT
to 0 Hz when the minimum output voltage of
the temperature sensor is reached. Potentiometer P1 is required
to calibrate the absolute accuracy of the AD654. The table in
Figure 34 illustrates the circuit element values for each of the
three sensors. The nominal offset voltage required for 0 Hz
output from the TMP35 is 50 mV; for the TMP36 and TMP37,
the offset voltage required is 100 mV. For the circuit values
shown, the output frequency transfer characteristic of the
circuit was set at 50 Hz/°C in all cases. At the receiving end, a
frequency-to-voltage converter (FVC) can be used to convert
the frequency back to a dc voltage for further processing. One
such FVC is the AD650.
For complete information about the AD650 and the AD654,
consult the individual data sheets for those devices.
DRIVING LONG CABLES OR HEAVY CAPACITIVE
LOADS
Although the TMP35/TMP36/TMP37 temperature sensors
can drive capacitive loads up to 10,000 pF without oscillation,
output voltage transient response times can be improved by
using a small resistor in series with the output of the temperature
sensor, as shown in Figure 35. As an added benefit, this resistor
forms a low-pass filter with the cable capacitance, which helps
to reduce bandwidth noise. Because the temperature sensor is
likely to be used in environments where the ambient noise level
can be very high, this resistor helps to prevent rectification by
the devices of the high frequency noise. The combination of this
resistor and the supply bypass capacitor offers the best protection.
TMP3x
0.1µF
GND
+V
S
750Ω
LONG CABLE OR
HEAVY CAPACITIVE
LOADS
V
OUT
00337-033
Figure 35. Driving Long Cables or Heavy Capacitive Loads
COMMENTARY ON LONG-TERM STABILITY
The concept of long-term stability has been used for many years
to describe the amount of parameter shift that occurs during
the lifetime of an IC. This is a concept that has been typically
applied to both voltage references and monolithic temperature
sensors. Unfortunately, integrated circuits cannot be evaluated
at room temperature (25°C) for 10 years or more to determine
this shift. As a result, manufacturers very typically perform
accelerated lifetime testing of integrated circuits by operating
ICs at elevated temperatures (between 125°C and 150°C) over a
shorter period of time (typically, between 500 and 1000 hours).
As a result of this operation, the lifetime of an integrated circuit
is significantly accelerated due to the increase in rates of reaction
within the semiconductor material.
Rev. H | Page 17 of 19
