Data Sheet TMP35/TMP36/TMP37
USING TMP35/TMP36/TMP37 SENSORS IN
REMOTE LOCATIONS
In many industrial environments, sensors are required to
operate in the presence of high ambient noise. These noise
sources take many forms, for example, SCR transients, relays,
radio transmitters, arc welders, and ac motors. They can also
be used at considerable distances from the signal conditioning
circuitry. These high noise environments are typically in the
form of electric fields, so the voltage output of the temperature
sensor can be susceptible to contamination from these noise
sources.
Figure 32 illustrates a way to convert the output voltage of a
TMP35/TMP36/TMP37 sensor into a current to be transmitted
down a long twisted pair shielded cable to a ground referenced
receiver. The temperature sensors are not capable of high output
current operation; thus, a standard PNP transistor is used to
boost the output current drive of the circuit. As shown in the
table in Figure 32, the values of R2 and R3 were chosen to
produce an arbitrary full-scale output current of 2 mA. Lower
values for the full-scale current are not recommended. The
minimum-scale output current produced by the circuit could be
contaminated by ambient magnetic fields operating in the near
vicinity of the circuit/cable pair. Because the circuit uses an
external transistor, the minimum recommended operating
voltage for this circuit is 5 V. To minimize the effects of EMI (or
RFI), both the circuit and the temperature sensor supply pins
are bypassed with good quality ceramic capacitors.
TWISTED PAIR
BELDEN TYPE 9502
OR EQUIVALENT
TMP3x
R2
R1
4.7kΩ
V
OUT
0.1µF
2N2907
0.01µF
GND
+V
S
5V
R3
V
OUT
SENSOR
R2 R3
TMP35 634
634
TMP36 887
887
TMP37 1k 1k
00337-030
Figure 32. Remote, 2-Wire Boosted Output Current Temperature Sensor
TEMPERATURE TO 4–20 mA LOOP TRANSMITTER
In many process control applications, 2-wire transmitters are
used to convey analog signals through noisy ambient environ-
ments. These current transmitters use a zero-scale signal current
of 4 mA, which can be used to power the signal conditioning
circuitry of the transmitter. The full-scale output signal in these
transmitters is 20 mA.
Figure 33 illustrates a circuit that transmits temperature inform-
ation in this fashion. Using a TMP35/TMP36/TMP37 as the
temperature sensor, the output current is linearly proportional
to the temperature of the medium. The entire circuit operates
from the 3 V output of the REF193. The REF193 requires no
external trimming because of its tight initial output voltage
tolerance and the low supply current of the TMP35/TMP36/
TMP37, the OP193, and the REF193. The entire circuit consumes
less than 3 mA from a total budget of 4 mA. The OP193 regulates
the output current to satisfy the current summation at the
noninverting node of the OP193. A generalized expression for
the KCL equation at Pin 3 of the OP193 is given by
×
+
×
×
=
R2
R3V
R1
R3TMP3x
R7
1
I
REF
OUT
For each temperature sensor, Table 5 provides the values for the
components P1, P2, and R1 to R4.
Table 5. Circuit Element Values for Loop Transmitter
Sensor R1 P1 R2 P2 R3 R4
TMP35 97.6 kΩ 5 kΩ 1.58 MΩ 100 kΩ 140 kΩ 56.2 kΩ
TMP36 97.6 kΩ 5 kΩ 931 kΩ 50 kΩ 97.6 kΩ 47 kΩ
TMP37 97.6 kΩ 5 kΩ 10.5 kΩ 500 Ω 84.5 kΩ 8.45 kΩ
The 4 mA offset trim is provided by P2, and P1 provides the
full-scale gain trim of the circuit at 20 mA. These two trims do
not interact because the noninverting input of the OP193 is
held at a virtual ground. The zero-scale and full-scale output
currents of the circuit are adjusted according to the operating
temperature range of each temperature sensor. The Schottky
diode, D1, is required in this circuit to prevent loop supply
power-on transients from pulling the noninverting input of the
OP193 more than 300 mV below its inverting input. Without
this diode, such transients can cause phase reversal of the
operational amplifier and possible latch-up of the transmitter.
The loop supply voltage compliance of the circuit is limited by
the maximum applied input voltage to the REF193; it is from
9 V to 18 V.
Rev. H | Page 15 of 19
