TMP01GBC AD [Analog Devices], TMP01GBC Datasheet - Page 13

• Manufacturer Part Number: TMP01GBC
• Description: Low Power, Programmable Temperature Controller
• Manufacturer: AD [Analog Devices]

REV. C
4 mA-20 mA Current Loop
Another, very common method of transmitting a signal over
long distances is to use a 4 mA-20 mA Loop, as shown in Fig-
ure 19. An advantage of using a 4 mA-20 mA loop is that the
accuracy of a current loop is not compromised by voltage drops
across the line. One requirement of 4 mA-20 mA circuits is that
the remote end must receive all of its power from the loop,
meaning that the circuit must consume less than 4 mA. Operat-
ing from +5 V, the quiescent current of the TMP01 is 500 A
max, and the OP90s is 20 A max, totaling less than 4 mA.
Although not shown, the open collector outputs and tempera-
ture setting pins can be connected to do any local control of
switching.
The current is proportional to the voltage on the VPTAT out-
put, 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:
The resulting temperature coefficient of the output current is
128 A/ C.
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
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 calcu-
lated, the value of R3 and R1 is determined by substituting
4 mA for I
values are shown in the circuit. The OP90 is chosen for this cir-
cuit because of its ability to operate on a single supply and its
100k
243k
I
OUT
R1
R3
OUT
R6
Figure 19. 4-20 mA Current Loop
1
and 1.165 V for VPTAT and solving. The final
39.2k
VPTAT
R2
1
4
R2
VREF
GND
2
3
OP90
TMP01
100k
R5
R5
7
4
VPTAT
OUT
–
V+
VREF
6
8
5
from 4 mA to 20 mA as
R3 R1
100
R6
2N1711
R3
1
+5V TO +13.2V
R
L
4–20mA
R5
R2
–13–
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-col-
lector digital output. A digital signal is very 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 suc-
cessfully transmitted.
A simple circuit to do this combines the TMP01 with an
AD654 VFC, as shown in Figure 20. The AD654 outputs a
square wave that is proportional to the dc input voltage accord-
ing to the following equation:
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, please consult the
AD654 data sheet. Finally, the AD650 can be used to accu-
rately convert the frequency back to a dc voltage on the receiv-
ing end.
R2
R1
R3
1
2
3
4
Figure 20. Temperature-to-Frequency Converter
VREF
GENERATOR
HYSTERESIS
TEMPERATURE
COMPARATOR
REFERENCE
SENSOR &
VOLTAGE
WINDOW
F
OUT
TMP01
VPTAT
10 (R1 R2) C
500
1.8k
VPTAT
8
7
6
5
R2
R1
V+
V
4
3
IN
V+
AD654
8
T
6
0.1 F
OSC
TMP01
5
C
T
7
2
1
V+
F
5k
OUT