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參數(shù)資料
| 型號： | ADT50 |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 溫度/濕度傳感器 |
| 英文描述： | Low Voltage SOT-23 Temperature Sensors(低壓溫度傳感器) |
| 中文描述： | 低壓SOT - 23封裝溫度傳感器（低壓溫度傳感器） |
| 文件頁數(shù)： | 10/12頁 |
| 文件大?。?/td> | 168K |
| 代理商： | ADT50 |
ADT45/ADT50
–10–
REV. 0
circuit ambient temperatures in the range of +20
°
C to +50
°
C.
Over a 250
°
C measurement temperature range, the thermocouple
produces an output voltage change of 10.151 mV. Since the
required circuit’s output full-scale voltage is 2.5 V, the gain of
the circuit is set to 246.3. Choosing R4 equal to 4.99 k
sets R5
equal to 1.22 M
. Since the closest 1% value for R5 is 1.21 M
, a
50 k
potentiometer is used with R5 for fine trim of the full-scale
output voltage. Although the OP193 is a superior single-supply,
micropower operational amplifier, its output stage is not rail-to-
rail; as such, the 0
°
C output voltage level is 0.1 V. If this circuit
were to be digitized by a single-supply ADC, the ADC’s common
should be adjusted accordingly to 0.1 V.
Using ADT 45/ADT 50 Sensors in Remote Locations
In many industrial environments, sensors are required to oper-
ate in the presence of high ambient noise. T hese noise sources
take on many forms; for example, SCR transients, relays, radio
transmitters, arc welders, ac motors, et cetera. Furthermore,
they may be used at considerable distance from the signal condi-
tioning circuitry. T hese high noise environments are very typi-
cally in the form of electric fields, so the voltage output of the
temperature sensor can be susceptible to contamination from
these noise sources.
Illustrated in Figure 23 is a way to convert the output voltage of
an ADT 45/ADT 50 sensor into a current to be transmitted down
a long twisted-pair shielded cable to a ground referenced receiver.
T he temperature sensors do not possess the capability of high
output current operation; thus, a garden variety PNP transistor
is used to boost the output current drive of the circuit. As shown
in the table, 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 because the minimum-
scale output current produced by the circuit could be contami-
nated by nearby ambient magnetic fields operating in the vicinity
of the circuit/cable pair. Because of the use of an external tran-
sistor, the minimum recommended operating voltage for this
circuit is +5 V. Note, to minimize the effects of EMI (or RFI)
both the circuit’s and the temperature sensor’s supply pins are
bypassed with good quality, ceramic capacitors.
TWISTED PAIR
BELDEN TYPE 9502
OR EQUIVALENT
ADT45/50
2N2907
0.1
m
F
R1
4.7k
V
0.01
m
F
+5V
V
OUT
R3
V
OUT
R2
GND
SENSOR
ADT45
ADT50
R2
634
887
R3
634
887
Figure 23. A Remote, Two-Wire Boosted Output Current
Temperature Sensor
A T emperature to 4 mA–20 mA Loop T ransmitter
In many process control applications, two-wire transmitters are
used to convey analog signals through noisy ambient environ-
ments. T hese current transmitters use a “zero-scale” signal
current of 4 mA that can be used to power the transmitter’s
signal conditioning circuitry. T he “full-scale” output signal in
these transmitters is 20 mA.
A circuit that transmits temperature information in this fashion is
illustrated in Figure 25. Using an ADT 45/ADT 50 as the tem-
perature sensor, the output current is linearly proportional to the
temperature of the medium. T he entire circuit operates from the
REF193’s +3 V output. T he REF193 requires no external trim-
ming for two reasons: (1) the REF193’s tight initial output
voltage tolerance and (2) the low supply current of the ADT 45/
ADT 50, the OP193 and the REF193. T he entire circuit con-
sumes less than 3 mA from a total budget of 4 mA. T he OP193
regulates the output current to satisfy the current summation at
the noninverting node of the OP193. A generalized expression for
the K CL equation at the OP193’s Pin 3 is given by:
I
OUT
=
1
R
7
×
ADT 45 /
ADT
50
×
R
3
R
1
+
V
REF
×
R
3
R
2
For each of the three temperature sensors, T able II provides the
values for each of the components, P1, P2 and R1–R4:
T able II. Circuit E lement Values for Loop T ransmitter
Sensor
R1(
V
)
P1(
V
)
R2(
V
)
P2(
V
) R3(
V
)
R4(
V
)
ADT 45
ADT 50
97.6 k
97.6 k
5 k
5 k
1.58 M
931 k
100 k
50 k
140 k
97.6 k
56.2 k
47 k
T he 4 mA offset trim is provided by P2, and P1 provides the
circuit’s full-scale gain trim at 20 mA. T hese two trims do not
interact because the noninverting input of the OP193 is held at
a virtual ground. T he zero-scale and full-scale output currents
of the circuit are adjusted according to the operating tempera-
ture range of each temperature sensor. T he 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 di-
ode, such transients could cause phase reversal of the opera-
tional amplifier and possible latchup of the transmitter. T he loop
supply voltage compliance of the circuit is limited by the maximum
applied input voltage to the REF193 and is from +9 V to +18 V.
A T emperature-to-Frequency Converter
Another common method of transmitting analog information
from a remote location is to convert a voltage to an equivalent in
the frequency domain. T his is readily done with any of the low
cost, monolithic voltage-to-frequency converters (VFCs) avail-
able. T hese VFCs feature a robust, open-collector output tran-
sistor for easy interfacing to digital circuitry. T he digital signal
produced by the VFC is less susceptible to contamination from
external noise sources and line voltage drops because the only
important information is the frequency of the digital signal. As
long as the conversions between temperature and frequency are
done accurately, the temperature data from the sensors can be
reliably transmitted.
T he circuit in Figure 24 illustrates a method by which the out-
puts of these temperature sensors can be converted to a fre-
quency using the AD654. T he output signal of the AD654 is a
square wave that is proportional to the dc input voltage across
Pins 4 and 3. T he transfer equation of the circuit is given by:
f
OUT
=
V
TMP
±
V
OFFSET
10
×
(
R
T
×
C
T
)
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