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AD596AH-AD597AH-AD597AR
Thermocouple Conditioner and Setpoint Controller
FUNCTIONAL BLOCK DIAGRAMREV.B
Thermocouple Conditioner and
Setpoint Controller
FEATURES
Low Cost
Operates with Type J (AD596) or Type K (AD597)
Thermocouples
Built-In Ice Point Compensation
Temperature Proportional Operation – 10 mV/8C
Temperature Setpoint Operation – ON/OFF
Programmable Switching Hysteresis
High Impedance Differential Input
GENERAL DESCRIPTIONThe 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 fromV to +30V, 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.
*. Patent No. 4,029,974.
PRODUCT HIGHLIGHTSThe AD596/AD597 provides cold junction compensation
and a high gain amplifier which can be used as a setpoint
comparator.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.Linearization not required for thermocouple temperatures
close to 175°C (+100°C to +540°C for AD596).Cold junction compensation is optimized for ambient tem-
peratures ranging from +25°C to +100°C.In the stand-alone mode, the AD596/AD597 produces an
output voltage that indicates its own temperature.
TO-100
SOIC
AD596/AD597–SPECIFICATIONS
(@ +608C and VS = 10 V, Type J (AD596), Type K (AD597) Thermocouple,
unless otherwise noted)NOTESThis is a measure of the deviation from ideal with a measuring thermocouple junction of 175°C and a chip temperature of 60°C. The ideal transfer function is given by:
AD596: VOUT = 180.57 × (Vm – Va + (ambient in °C) × 53.21μV/°C + 235μV)
AD597: VOUT = 245.46 × (Vm – Va + (ambient in °C) × 41.27μV/°C – 37μV)
where Vm, and Va represent the measuring and ambient temperatures and are taken from the appropriate J or K thermocouple table. The ideal transfer function minimizes the
error over the ambient temperature range of 25°C to 100°C with a thermocouple temperature of approximately 175°C.2Defined as the slope of the line connecting the AD596/AD597 CJC errors measured at 25°C and 100°C ambient temperature.Pin 6 shorted to Pin 7.Current Sink Capability in single supply configuration is limited to current drawn to ground through a 50kΩ resistor at output voltages below 2.5V.Alarm function available on H package option only.
Specifications subject to change without notice.
Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min and max
specifications are guaranteed, although only those shown in boldface are tested on all production units.
ORDERING GUIDE*Consult factory for availability.
Table I.Output Voltage vs. Thermocouple Temperature (Ambient +608C, VS = –5 V, +15 V)
AD596/AD597
TEMPERATURE PROPORTIONAL OUTPUT MODEThe AD596/AD597 can be used to generate a temperature
proportional output of 10mV/°C when operated with J and K
type thermocouples as shown in Figure 1. Thermocouples pro-
duce low level output voltages which are a function of both the
temperature being measured and the reference or cold junction
temperature. The AD596/AD597 compensates for the cold
junction temperature and amplifies the thermocouple signal to
produce a high level 10mV/°C voltage output which is a func-
tion only of the temperature being measured. The temperature
stability of the part indicates the sensitivity of the output voltage
to changes in ambient or device temperatures. This is typically
0.02°C/°C over the +25°C to +100°C recommended ambient
temperature range. The parts will operate over the extended
ambient temperature ranges from –55°C to +125°C, but ther-
mocouple nonlinearity at the reference junction will degrade the
temperature stability over this extended range. Table I is a list of
ideal AD596/AD597 output voltages as a function of Celsius
temperature for type J and K ANSI standard thermocouples
with package and reference junction at 60°C. As is normally the
case, these outputs are subject to calibration and temperature
sensitivity errors. These tables are derived using the ideal trans-
fer functions:
AD596 output = (Type J voltage + 301.5μV) × 180.57
AD597 output = (Type K voltage) × 245.46
Figure 1. Temperature Proportional Output Connection
The offsets and gains of these devices have been laser trimmed
to closely approximate thermocouple characteristics over mea-
surement temperature ranges centered around 175°C with the
AD596/AD597 at an ambient temperature between 25°C and
100°C. This eliminates the need for additional gain or offset
adjustments to make the output voltage read:
VOUT = 10mV/°C × (thermocouple temperature in °C) (within
specified tolerances).
Excluding calibration errors, the above transfer function is accu-
rate to within 1°C from +80°C to +550°C for the AD596 and
–20°C to +350°C for the AD597. The different temperature
ranges are due to the differences in J and K type thermocouple
curves.
European DIN FE-CuNi thermocouple vary slightly from ANSI
type J thermocouples. Table I does not apply when these types
of thermocouples are used. The transfer functions given previ-
ously and a thermocouple table should be used instead.
Figure 1 also shows an optional trimming network which can be
used to change the device’s offset voltage. Injecting or sinking
200nA from Pin 3 will offset the output approximately 10mV
(1°C).
The AD596/AD597 can operate from a single supply from 5V
to 36V or from split supplies totalling 36V or less as shown.
Since the output can only swing to within 2V of the positive
supply, the usable measurement temperature range will be re-
stricted when positive supplies less than 15V for the AD597
and 10V for the AD596 are used. If the AD596/AD597 is to be
used to indicate negative Celsius temperatures, then a negative
supply is required.
Common-mode voltages on the thermocouple inputs must
remain within the common-mode voltage range of the AD596/
AD597, with a return path provided for the bias currents. If the
thermocouple is not remotely grounded, then the dotted line
connection shown in Figure 1 must be made to one of the ther-
mocouple inputs. If there is no return path for the bias currents,
the input stage will saturate, causing erroneous output voltages.
In this configuration, the AD596/AD597 H package option has
circuitry which detects the presence of an open thermocouple. If
the thermocouple loop becomes open, one or both of the inputs
to the device will be deprived of bias current causing the output
to saturate. It is this saturation which is detected internally and
used to activate the alarm circuitry. The output of this feature
has a flexible format which can be used to source or sink up tomA of current. The collector (+ALM) should not be allowed
to become more positive than (–VS + 36V), however, it may be
permitted to be more positive than +VS. The emitter voltage
(–ALM) should be constrained such that it does not become
more positive than 4V below +VS. If the alarm feature is not
used, this pin should be connected to Pins 4 or 5 as shown in
Figure 1. The alarm function is unavailable on the AR package
option.
SETPOINT CONTROL MODEThe 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 × 9.6mV/°C + 42mV
AD597 Setpoint Voltage = °C × 10.1mV/°C – 9.1mV
The input impedance of the setpoint pin of the AD596/AD597
is approximately 50kΩ. The temperature coefficient of this
resistance is ±15ppm/°C. Therefore, the 100ppm/°C 5kΩ 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.
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 200nA of current
injected into Pin 3 when the output switches will cause about
1°C of hysteresis; that is:
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 – 4V) for proper operation
of the rest of the circuit.
STAND-ALONE TEMPERATURE TRANSDUCERThe AD596/AD597 may be configured as a stand-alone Celsius
thermometer as shown in Figure 3.
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 42mV offset
with the AD596 causing it to read slightly high when used in
this mode.
THERMOCOUPLE CONNECTIONSThe 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.
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%
AD596/AD597
SINGLE AND DUAL SUPPLY CONNECTIONSIn the single supply configuration as used in the setpoint con-
troller of Figure 2, any convenient voltage from +5V to +36V
may be used, with self-heating errors being minimized at lower
supply levels. In this configuration, the –VS connection at Pin 5
is tied to ground. Temperatures below zero can be accommo-
dated in the single supply setpoint mode, but not in the single
supply temperature measuring mode (Figure 1 reconnected for
single supply). Temperatures below zero can only be indicated
by a negative output voltage, which is impossible in the single
supply mode.
Common-mode voltages on the thermocouple inputs must
remain below the positive supply, and not more than 0.15V
more negative than the minus supply. In addition, a return path
for the input bias currents must be provided. If the thermo-
couple is not remotely grounded, then the dotted line connec-
tions in Figures 1 and 2 are mandatory.
STABILITY OVER TEMPERATUREThe AD596/AD597 is specified for a maximum error of ±4°C at
an ambient temperature of 60°C and a measuring junction
temperature at 175°C. The ambient temperature stability is
specified to be a maximum of 0.05°C/°C. In other words, for
every degree change in the ambient temperature, the output will
change no more than 0.05 degrees. So, at 25°C the maximum
deviation from the temperature-voltage characteristic of Table I
is ±5.75°C, and at 100°C it is ±6°C maximum (see Figure 5). If
the offset error of ±4°C is removed with a single offset adjust-
ment, these errors will be reduced to ±1.75°C and ±2°C max.
The optional trim circuit shown in Figure 1 demonstrates how
the ambient offset error can be adjusted to zero.
Figure 5.Drift Error vs. Temperature
THERMAL ENVIRONMENTAL EFFECTSThe inherent low power dissipation of the AD596/AD597 keeps
self-heating errors to a minimum. However, device output is
capable of delivering ±5mA to an external load and the alarm
circuitry can supply up to 20mA. Since the typical junction to
ambient thermal resistance in free air is 150°C/W, significant
temperature difference between the package pins (where the
reference junction is located) and the chip (where the cold junc-
tion temperature is measured and then compensated) can exist
when the device is operated in a high dissipation mode. These
temperature differences will result in a direct error at the out-
put. In the temperature proportional mode, the alarm feature
will only activate in the event of an open thermocouple or sys-
tem transient which causes the device output to saturate.
Self-Heating errors will not effect the operation of the alarm but
two cases do need to be considered. First, after a fault is cor-
rected and the alarm is reset, the AD596/AD597 must be al-
lowed to cool before readings can again be accurate. This can
take 5 minutes or more depending upon the thermal environ-
ment seen by the device. Second, the junction temperature of
the part should not be allowed to exceed 150°C. If the alarm
circuit of the AD596/AD597 is made to source or sink 20mA
with 30V across it, the junction temperature will be 90°C above
ambient causing the die temperature to exceed 150°C when
ambient is above 60°C. In this case, either the load must be
reduced, or a heat sink used to lower the thermal resistance.
TEMPERATURE READOUT AND CONTROLFigure 6 shows a complete temperature indication and control
system based on the AD596/AD597. Here the AD596/AD597 is
being used as a closed-loop thermocouple signal conditioner
and an external op amp is used to implement setpoint. This has
two important advantages. It provides a high level (10mV/°C)
output for the A/D panel meter and also preserves the alarm
function for open thermocouples.
The A/D panel meter can easily be offset and scaled as shown to
read directly in degrees Fahrenheit. If a two temperature cali-
bration scheme is used, the dominant residual errors will arise
from two sources; the ambient temperature rejection (typically
±2°C over a 25°C to 100°C range) and thermocouple nonlin-
earity typical +1°C from 80°C to 550°C for type J and +1°C
from –20°C to 350°C for type K.
An external voltage reference is used both to increase the stabil-
ity of the A/D converter and supply a stable reference for the
setpoint voltage.
A traditional requirement for the design of setpoint control
thermocouple systems has been to configure the system such
that the appropriate action is taken in the event of an open
thermocouple. The open thermocouple alarm pin with its flex-
ible current-limited output format supports this function when
the part operates in the temperature proportional mode. In
addition, if the thermocouple is not remotely grounded, it is
possible to program the device for either a positive or negative
full-scale output in the event of an open thermocouple. This is
done by connecting the bias return resistor directly to Pin 1 if a
high output voltage is desired to indicate a fault condition. Al-
ternately, if the bias return is provided on the thermocouple lead
connected to Pin 2, an open circuit will result in an output low
reading. Figure 6 shows the ground return connected to Pin 1
so that if the thermocouple fails, the heater will remain off. At
the same time, the alarm circuit lights the LED signalling the
need to service the thermocouple. Grounding Pin 2 would lead
to low output voltage saturation, and in this circuit would result
in apotentially dangerous thermal runaway under fault conditions.
