TMP17G View Datasheet(PDF) - Analog Devices
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TMP17G Datasheet PDF : 8 Pages
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TMP17
A variable temperature controlling thermostat can easily be built
using the TMP17 in the circuit of Figure 18.
+15V
10V
TMP17
10kΩ
C
REF01E
RHIGH
62.7kΩ
RSET
10kΩ
AD790
COMPARATOR
RHYST
RPULL-UP
TEMP > SETPOINT
OUTPUT HIGH
TEMP < SETPOINT
OUTPUT LOW
RLOW
27.3kΩ
(OPTIONAL)
C
control which row of sensors are being measured. The maxi-
mum number of TMP17s which can be used is the product of
the number of channels of the decoder and mux.
An example circuit controlling 80 TMP17s is shown in Figure
20. A 7-bit digital word is all that is required to select one of
the sensors. The enable input of the multiplexer turns all the
sensors off for minimum dissipation while idling.
COLUMN
SELECT
+15V
4028 BCD TO DECIMAL DECODER
ROW
SELECT
VOUT
10kΩ
Figure 18. Variable Temperature Thermostat
RHIGH and RLOW determine the limits of temperature controlled
by the potentiometer RSET. The circuit shown operates over the
temperature range Ϫ25°C to ϩ105°C. The reference maintains
a constant set point voltage and insures that approximately 7 V
appears across the sensor. If it is necessary to guardband for
extraneous noise, hysteresis can be added by tying a resistor
from the output to the ungrounded end of RLOW.
Multiple remote temperatures can be measured using several
TMP17s with a CMOS multiplexer or a series of 5 V logic gates
because of the device’s current-mode output and supply-voltage
compliance range. The on-resistance of a FET switch or output
impedance of a gate will not affect the accuracy, as long as 4 V
is maintained across the transducer. Muxes and logic driving
circuits should be chosen to minimize leakage current related
errors. Figure 19 illustrates a locally controlled mux switching
the signal current from several remote TMP17s. CMOS or TTL
gates can also be used to switch the TMP17 supply voltages,
with the multiplexed signal being transmitted over a single
twisted pair to the load.
+15V
–15V
T8
T2
T1
REMOTE
TMP17s
AD7501
D
S1
ED
CR
S2
OI
DV
S8
EE
RR
/
TTL DTL TO
CMOS I/O
VOUT
10kΩ
EN
CHANNEL
SELECT
+15V
–15V
EN
80 – TMP17s
Figure 20. Matrix Multiplexer
To convert the TMP17 output to °C or °F a single inexpensive
reference and op amp can be used as shown in Figure 21.
Although this circuit is similar to the two temperature trim
circuit shown in Figure 11, two important differences exist.
First, the gain resistor is fixed alleviating the need for an
elevated temperature trim. Acceptable accuracy can be achieved
by choosing an inexpensive resistor with the correct tolerance.
Second, the TMP17 calibration error can be trimmed out at a
known convenient temperature (i.e., room temperature) with a
single pot adjustment. This step is independent of the gain
selection.
+5V
REF43
2.5V
ROFFSET
R
RCAL
ROFFSET/RGAIN
RGAIN
OP196
ROFFSET
C ≈ 9.1kΩ
F ≈ 9.8kΩ
RGAIN
100kΩ
180kΩ
VOUT = 100mV/(oC OR oF)
TMP17
V–
Figure 21. Celsius or Fahrenheit Thermometer
Figure 19. Remote Temperature Multiplexing
To minimize the number of muxes required when a large
number of TMP17s are being used, the circuit can be config-
ured in a matrix. That is, a decoder can be used to switch the
supply voltage to a column of TMP17s while a mux is used to
REV. 0
–7–
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