AD596/AD597
SETP O INT CO NTRO L MO D E
T he AD596/AD597 can be connected as a setpoint controller as
shown in Figure 2. T he thermocouple voltage is cold junction
compensated, amplified, and compared to an external setpoint
voltage. T he relationship between setpoint voltage and tempera-
ture is given in T able I. If the temperature to be controlled is
within the operating range (–55°C to +125°C) of the device, it
can monitor its own temperature by shorting the inputs to
ground. T he setpoint voltage with the thermocouple inputs
grounded is given by the expressions:
+V
S
–
G
+
0.01F
ICE
POINT
COMP
–
+
+A
G
+
V
AD596/
AD597*
OUT
9.6mV/؇C
AD596 Setpoint Voltage = °C × 9.6 mV/°C + 42 mV
AD597 Setpoint Voltage = °C × 10.1 mV/°C – 9.1 mV
*H PACKAGE PINOUT SHOWN
0.01F
T he input impedance of the setpoint pin of the AD596/AD597
is approximately 50 kΩ. T he temperature coefficient of this
resistance is ±15 ppm/°C. T herefore, the 100 ppm/°C 5 kΩ pot
shown in Figure 2 will only introduce an additional ±1°C degra-
dation of temperature stability over the +25°C to +100°C ambi-
ent temperature range.
–V
S
Figure 3. Stand-Alone Tem perature Transducer
Tem perature Proportional Output Connection
Simply omit the thermocouple and connect the inputs (Pins 1
and 2) to common. T he output will now reflect the compensa-
tion voltage and hence will indicate the AD596/AD597 tem-
perature. In this three terminal, voltage output, temperature
sensing mode, the AD596/AD597 will operate over the full
extended –55°C to +125°C temperature range. T he output
scaling will be 9.6 mV per °C with the AD596 and 10.1 mV per
°C with the AD597. Additionally there will be a 42 mV offset
with the AD596 causing it to read slightly high when used in
this mode.
TEMPERATURE
CONTROLLED
REGION
CONSTANTAN
(ALUMEL)
TEMPERATURE
COMPARATOR
OUTPUT
0.01F
+V
IRON
(CHROMEL)
AD596/
AD597*
V
REF
SET-
POINT
VOLTAGE
5k⍀
100ppm/؇C
SET-
POINT
VOLTAGE
R
TH ERMO CO UP LE CO NNECTIO NS
HYSTERESIS
(OPTIONAL)
T he connection of the thermocouple wire and the normal wire
or printed circuit board traces going to the AD596/AD597
forms an effective reference junction as shown in Figure 4. T his
junction must be kept at the same temperature as the AD596/
AD597 for the internal cold junction compensation to work
properly. Unless the AD596/AD597 is in a thermally stable
enclosure, the thermocouple leads should be brought in directly
to Pins 1 and 2.
HEATER
DRIVER
*H PACKAGE PINOUT SHOWN
Figure 2. Setpoint Control Mode
Switching hysteresis is often used in setpoint systems of this type
to provide noise immunity and increase system reliability. By
reducing the frequency of on-off cycling, mechanical component
wear is reduced leading to enhanced system reliability. T his can
easily be implemented with a single external resistor between
Pins 7 and 3 of the AD596/AD597. Each 200 nA of current
injected into Pin 3 when the output switches will cause about
1°C of hysteresis; that is:
REFERENCE JUNCTION
LIMITING RESISTOR
CONSTANTAN
(ALUMEL)
TO
LED
0.01F
IRON
+V
S
(CHROMEL)
AD596/
AD597*
NOTE:
VOUT
1
A BIAS RETURN PATH
FROM PINS 1 AND 2
OF LESS THAN 1k⍀
IMPEDANCE MUST BE
PROVIDED.
RHYST (Ω) =
×
200 nA °CHYST
V
OUT
In the setpoint configuration, the AD596/AD597 output is
saturated at all times, so the alarm transistor will be ON regard-
less of whether there is an open circuit or not. However, –ALM
must be tied to a voltage below (+VS – 4 V) for proper operation
of the rest of the circuit.
0.01F
*H PACKAGE PINOUT SHOWN
–V
GND
S
STAND -ALO NE TEMP ERATURE TRANSD UCER
T he AD596/AD597 may be configured as a stand-alone Celsius
thermometer as shown in Figure 3.
Figure 4. PCB Connections
T o ensure secure bonding, the thermocouple wire should be
cleaned to remove oxidization prior to soldering. Noncorrosive
resin flux is effective with iron, constantan, chromel, and
alumel, and the following solders: 95% tin–5% silver, or 90%
tin–10% lead.
REV. B
–5–
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