AD592
REV.
–7–
The circuit shown can be optimized for any ambient tempera-
ture range or thermocouple type by simply selecting the correct
value for the scaling resistor – R. The AD592 output (1 µA/K)
times R should approximate the line best fit to the thermocouple
curve (slope in V/°C) over the most likely ambient temperature
range. Additionally, the output sensitivity can be chosen by
selecting the resistors R
G1
and R
G2
for the desired noninverting
gain. The offset adjustment shown simply references the AD592
to °C. Note that the TC’s of the reference and the resistors are
the primary contributors to error. Temperature rejection of 40
to 1 can be easily achieved using the above technique.
Although the AD592 offers a noise immune current output, it is
not compatible with process control/industrial automation cur-
rent loop standards. Figure 12 is an example of a temperature to
4–20 mA transmitter for use with 40 V, 1 kΩ systems.
In this circuit the 1 µA/K output of the AD592 is amplified to
1 mA/°C and offset so that 4 mA is equivalent to 17°C and
20 mA is equivalent to 33°C. Rt is trimmed for proper reading
at an intermediate reference temperature. With a suitable choice
of resistors, any temperature range within the operating limits of
the AD592 may be chosen.
AD592
AD581
35.7kΩ
10mV/
o
C
10kΩ
12.7kΩ
5kΩ 500Ω
+20V
–20V
V
T
10Ω
C
R
T
5kΩ
1mA/
o
C
208
17°C ≈ 4mA
33°C ≈ 20µA
Figure 12. Temperature to 4–20 mA Current Transmitter
Reading temperature with an AD592 in a microprocessor based
system can be implemented with the circuit shown in Figure 13.
AD1403
950Ω
9kΩ
1kΩ
100Ω
+5V
AD592
SPAN
TRIM
CENTER
POINT
TRIM
FORMAT
BPO/UPO
200Ω
µP CONTROL
GND
V
IN
HI
V
I
HI
N
V
I
LO
N
V
I
LO
N
8 BITS
OUT
AD670
ADCPORT
R/W CS CE
V
CC
Figure 13. Temperature to Digital Output
By using a differential input A/D converter and choosing the
current to voltage conversion resistor correctly, any range of
temperatures (up to the 130°C span the AD592 is rated for)
centered at any point can be measured using a minimal number
of components. In this configuration the system will resolve up
to 1°C.
A variable temperature controlling thermostat can easily be built
using the AD592 in the circuit of Figure 14.
AD592
10kΩ
R
HYST
R
PULL-UP
+15V
COMPARATOR
(OPTIONAL)
C
R
HIGH
62.7kΩ
R
SET
10kΩ
C
TEMP > SETPOINT
OUTPUT HIGH
TEMP < SETPOINT
OUTPUT LOW
R
LOW
27.3kΩ
AD581
Figure 14. Variable Temperature Thermostat
R
HIGH
and R
LOW
determine the limits of temperature controlled
by the potentiometer R
SET
. The circuit shown operates over the
full temperature range (–25°C to +105°C) the AD592 is rated
for. 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 R
LOW.
Multiple remote temperatures can be measured using several
AD592s 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. MUXs and logic driving
circuits should be chosen to minimize leakage current related
errors. Figure 15 illustrates a locally controlled MUX switching
the signal current from several remote AD592s. CMOS or TTL
gates can also be used to switch the AD592 supply voltages,
with the multiplexed signal being transmitted over a single
twisted pair to the load.
AD7501
D
E
C
O
D
E
R
/
D
R
I
V
E
R
T
8
T
2
T
1
REMOTE
AD592s
S1
S2
S8
E
N
TTL DTL TO
CMOS I/O
CHANNEL
SELECT
+15V –15V
V
OUT
10kΩ
Figure 15. Remote Temperature Multiplexing
