AD596 Datasheet PDF - Analog Devices
| Part Number | AD596 | |
| Description | Thermocouple Conditioner and Setpoint Controller | |
| Manufacturers | Analog Devices | |
| Logo |  |
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a
Thermocouple Conditioner and
Setpoint Controller
AD596*/AD597*
FEATURES
Low Cost
Operates with Type J (AD596) or Type K (AD597)
Thermocouples
Built-In Ice Point Compensation
Temperature Proportional Operation – 10 mV/؇C
Temperature Setpoint Operation – ON/OFF
Programmable Switching Hysteresis
High Impedance Differential Input
GENERAL DESCRIPTION
The AD596/AD597 is a monolithic temperature setpoint con-
troller that has been optimized for use at elevated temperatures
such as those found in oven control applications. The device
cold junction compensates and amplifies a type J or K thermo-
couple input to derive an internal signal proportional to tem-
perature. The internal signal is then compared with an externally
applied setpoint voltage to yield a low impedance switched output
voltage. Dead-Band or switching hysteresis can be programmed
using a single external resistor. Alternately, the AD596/AD597 can
be configured to provide a voltage output (10 mV/°C) directly from
a type J or K thermocouple signal. It can also be used as a stand-
alone voltage output temperature sensor.
The AD596/AD597 can be powered with a single supply from
+5 V to +30 V, or dual supplies up to a total span of 36 V.
Typical quiescent supply current is 160 µA, which minimizes
self-heating errors.
The AD596/AD597 H package option includes a thermocouple
failure alarm that indicates an open thermocouple lead when
operated in the temperature proportional measurement mode.
The alarm output has a flexible format which can be used to
drive relays, LEDs or TTL logic.
The device is packaged in a reliability qualified, cost effective
10-pin metal can or SOIC and is trimmed to operate over an
ambient temperature range from +25°C to +100°C. Operation
over an extended ambient temperature range is possible with
slightly reduced accuracy. The AD596 will amplify thermo-
couple signals covering the entire –200°C to +760°C tempera-
ture range recommended for type J thermocouples while the
AD597 can accommodate –200°C to +1250°C type K inputs.
The AD596/AD597 has a calibration accuracy of ±4°C at an
ambient temperature of 60°C and an ambient temperature
stability specification of 0.05°C/°C from +25°C to +100°C. If
higher accuracy, or a lower ambient operating temperature is
required, either the AD594 (J thermocouple) or AD595 (K
thermocouple) should be considered.
*Protected by U.S. Patent No. 4,029,974.
FUNCTIONAL BLOCK DIAGRAM
TO-100
–ALM
–IN +ALM
+IN
ICE
POINT
COMP
HYS
–
G
+
–
G + +A
+
AD596/
AD597
V+
VOUT
GND
FB
V–
SOIC
+IN 1
HYS 2
AD597
+–
G
+ +A
GND 3
G
+–
8 –IN
7 V+
6 VOUT
V– 4
ICE POINT
COMP
TOP VIEW
(Not to Scale)
5 FB
PRODUCT HIGHLIGHTS
1. The AD596/AD597 provides cold junction compensation
and a high gain amplifier which can be used as a setpoint
comparator.
2. The input stage of the AD596/AD597 is a high quality in-
strumentation amplifier that allows the thermocouple to float
over most of the supply voltage range.
3. Linearization not required for thermocouple temperatures
close to 175°C (+100°C to +540°C for AD596).
4. Cold junction compensation is optimized for ambient tem-
peratures ranging from +25°C to +100°C.
5. In the stand-alone mode, the AD596/AD597 produces an
output voltage that indicates its own temperature.
REV. B
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1998
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AD596/AD597
SETPOINT CONTROL MODE
The AD596/AD597 can be connected as a setpoint controller as
shown in Figure 2. The thermocouple voltage is cold junction
compensated, amplified, and compared to an external setpoint
voltage. The relationship between setpoint voltage and tempera-
ture is given in Table 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. The setpoint voltage with the thermocouple inputs
grounded is given by the expressions:
AD596 Setpoint Voltage = °C × 9.6 mV/°C + 42 mV
AD597 Setpoint Voltage = °C × 10.1 mV/°C – 9.1 mV
The input impedance of the setpoint pin of the AD596/AD597
is approximately 50 kΩ. The temperature coefficient of this
resistance is ± 15 ppm/°C. Therefore, 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.
TEMPERATURE
CONTROLLED CONSTANTAN
REGION
(ALUMEL)
IRON
(CHROMEL)
AD596/
AD597*
TEMPERATURE
COMPARATOR
0.01F OUTPUT
+V
SET-
VREF
POINT
VOLTAGE
SET-
POINT
5k⍀
100ppm/؇C
R VOLTAGE
HYSTERESIS
(OPTIONAL)
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. This 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:
RHYST
(Ω) =
VOUT
200 nA
×
1
°CHYST
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.
STAND-ALONE TEMPERATURE TRANSDUCER
The AD596/AD597 may be configured as a stand-alone Celsius
thermometer as shown in Figure 3.
ICE
POINT
COMP
–
G
+
–
G + +A
+
AD596/
AD597*
+VS
0.01F
VOUT
9.6mV/؇C
*H PACKAGE PINOUT SHOWN
0.01F
–VS
Figure 3. Stand-Alone Temperature Transducer
Temperature Proportional Output Connection
Simply omit the thermocouple and connect the inputs (Pins 1
and 2) to common. The 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. The 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.
THERMOCOUPLE CONNECTIONS
The 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. This
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.
REFERENCE JUNCTION
CONSTANTAN
(ALUMEL)
IRON
(CHROMEL)
NOTE:
A BIAS RETURN PATH
FROM PINS 1 AND 2
OF LESS THAN 1k⍀
IMPEDANCE MUST BE
PROVIDED.
AD596/
AD597*
LIMITING RESISTOR
TO
LED
0.01F
+VS
VOUT
0.01F
*H PACKAGE PINOUT SHOWN
GND
–VS
Figure 4. PCB Connections
To 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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