TMP01
Rev. E | Page 14 of 20
The current is proportional to the voltage on the VPTAT
output, and is calibrated to 4 mA at a temperature of −40°C, to
20 mA for +85°C. The main equation governing the operation
of this circuit gives the current as a function of VPTAT
⎟
⎠
⎞
⎜
⎝
⎛
⎟
⎠
⎞
⎜
⎝
⎛
+
+
×
−
×
=
2
5
1
13
3
2
5
6
1
R
R
RR
RVREF
R
RVPTAT
R
I
OUT
The resulting temperature coefficient of the output current is
128 A/°C.
5
8
1
4
2N1711
VREF
GND
V+
VPTAT
TMP01
OP90
4–20mA
5V TO 13.2V
7
4
3
6
2
R
L
R6
100Ω
R1
243kΩ
R2
39.2kΩ
R3
100kΩ
R5
100kΩ
00333-027
Figure 27. 4mA to 20 mA Current Loop
To determine the resistor values in this circuit, first note that
VREF remains constant over temperature. Thus, the ratio of
R5 over R2 must give a variation of I
OUT
from 4 mA to 20 mA
as VPTAT varies from 1.165 V at −40°C to 1.79 V at +85°C.
The absolute value of the resistors is not important, only the
ratio. For convenience, 100 k is chosen for R5. Once R2 is
calculated, the value of R3 and R1 is determined by substituting
4 mA for I
OUT
and 1.165 V for VPTAT and solving. The final
values are shown in the circuit. The OP90 is chosen for this
circuit because of its ability to operate on a single supply and its
high accuracy. For initial accuracy, a 10 k trim potentiometer
can be included in series with R3, and the value of R3 lowered
to 95 k. The potentiometer should be adjusted to produce an
output current of 12.3 mA at 25°C.
TEMPERATURE-TO-FREQUENCY CONVERTER
Another common method of transmitting analog information
is to convert a voltage to the frequency domain. This is easily
done with any of the low cost monolithic voltage-to-frequency
converters (VFCs) available, which feature a robust, open-
collector digital output. A digital signal is immune to noise
and voltage drops because the only important information is
the frequency. As long as the conversions between temperature
and frequency are done accurately, the temperature data can be
successfully transmitted.
A simple circuit to do this combines the TMP01 with an AD654
VFC, as shown in Figure 28. The AD654 outputs a square wave
that is proportional to the dc input voltage according to the
following equation:
T
IN
OUT
CRR
V
F
)21(10 +
=
By simply connecting the VPTAT output to the input of the
AD654, the 5 mV/°C temperature coefficient gives a sensitivity
of 25 Hz/°C, centered around 7.5 kHz at 25°C. The trimming
resistor R2 is needed to calibrate the absolute accuracy of the
AD654. For more information on that part, consult the AD654
data sheet. Finally, the AD650 can be used to accurately convert
the frequency back to a dc voltage on the receiving end.
TEMPERATURE
SENSOR AND
VOLTAGE
REFERENCE
VREF
VPTAT
VPTAT
1
2
3
4
8
7
6
5
HYSTERESIS
GENERATOR
WINDOW
COMPARATOR
TMP01
R1
R2
R3
+
F
OUT
V+
V+
4
3
1
2
5
AD654
OSC
78
6
R1
1.8kΩ
R2
500Ω
5kΩ
C
T
0.1µF
00333-028
Figure 28. Temperature-to-Frequency Converter
