AD594AQ ,Monolithic Thermocouple Amplifiers with Cold Junction Compensationapplications.to produce a high level (10 mV/
AD594AD-AD594AQ-AD594CD-AD594CQ-AD595AD-AD595AQ-AD595CD-AD595CQ-AD595CQ.
Monolithic Thermocouple Amplifiers with Cold Junction Compensation
FUNCTIONAL BLOCK DIAGRAM
+IN+C+TCOM–T–CV–
–IN–ALM+ALMV+COMPVOFBMonolithic Thermocouple Amplifiers
with Cold Junction Compensation
FEATURES
Pretrimmed for Type J (AD594) or
Type K (AD595) Thermocouples
Can Be Used with Type T Thermocouple Inputs
Low Impedance Voltage Output: 10 mV/8C
Built-In Ice Point Compensation
Wide Power Supply Range: +5 V to 615 V
Low Power: <1 mW typical
Thermocouple Failure Alarm
Laser Wafer Trimmed to 18C Calibration Accuracy
Setpoint Mode Operation
Self-Contained Celsius Thermometer Operation
High Impedance Differential Input
Side-Brazed DIP or Low Cost Cerdip
PRODUCT DESCRIPTIONThe AD594/AD595 is a complete instrumentation amplifier and
thermocouple cold junction compensator on a monolithic chip.
It combines an ice point reference with a precalibrated amplifier
to produce a high level (10 mV/°C) output directly from a ther-
mocouple signal. Pin-strapping options allow it to be used as a
linear amplifier-compensator or as a switched output setpoint
controller using either fixed or remote setpoint control. It can
be used to amplify its compensation voltage directly, thereby
converting it to a stand-alone Celsius transducer with a low
impedance voltage output.
The AD594/AD595 includes a thermocouple failure alarm that
indicates if one or both thermocouple leads become open. The
alarm output has a flexible format which includes TTL drive
capability.
The AD594/AD595 can be powered from a single ended supply
(including +5 V) and by including a negative supply, tempera-
tures below 0°C can be measured. To minimize self-heating, an
unloaded AD594/AD595 will typically operate with a total sup-
ply current 160 mA, but is also capable of delivering in excess of5 mA to a load.
The AD594 is precalibrated by laser wafer trimming to match
the characteristic of type J (iron-constantan) thermocouples and
the AD595 is laser trimmed for type K (chromel-alumel) inputs.
The temperature transducer voltages and gain control resistors
are available at the package pins so that the circuit can be
recalibrated for the thermocouple types by the addition of two
or three resistors. These terminals also allow more precise cali-
bration for both thermocouple and thermometer applications.
The AD594/AD595 is available in two performance grades. The
C and the A versions have calibration accuracies of –1°C and3°C, respectively. Both are designed to be used from 0°C to
+50°C, and are available in 14-pin, hermetically sealed, side-
brazed ceramic DIPs as well as low cost cerdip packages.
PRODUCT HIGHLIGHTSThe AD594/AD595 provides cold junction compensation,
amplification, and an output buffer in a single IC package.Compensation, zero, and scale factor are all precalibrated by
laser wafer trimming (LWT) of each IC chip.Flexible pinout provides for operation as a setpoint control-
ler or a stand-alone temperature transducer calibrated in
degrees Celsius.Operation at remote application sites is facilitated by low
quiescent current and a wide supply voltage range +5 V to
dual supplies spanning 30 V.Differential input rejects common-mode noise voltage on the
thermocouple leads.
REV.C
AD594/AD595–SPECIFICATIONSTEMPERATURE MEASUREMENT
AMPLIFIER CHARACTERISTICS
ALARM CHARACTERISTICS
POWER REQUIREMENTS
NOTESCalibrated for minimum error at +25°C using a thermocouple sensitivity of 51.7 mV/°C. Since a J type thermocouple deviates from this straight line approximation, the AD594 will normally
read 3.1 mV when the measuring junction is at 0°C. The AD595 will similarly read 2.7 mV at 0°C.Defined as the slope of the line connecting the AD594/AD595 errors measured at 0°C and 50°C ambient temperature.
3Pin 8 shorted to Pin 9.Current Sink Capability in single supply configuration is limited to current drawn to ground through a 50 kW resistor at output voltages below 2.5 V.
5–VS must not exceed –16.5 V.
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.
Specifications subject to change without notice.
(@ +258C and VS = 5 V, Type J (AD594), Type K (AD595) Thermocouple,
unless otherwise noted)
INTERPRETING AD594/AD595 OUTPUT VOLTAGESTo achieve a temperature proportional output of 10 mV/°C and
accurately compensate for the reference junction over the rated
operating range of the circuit, the AD594/AD595 is gain trimmed
to match the transfer characteristic of J and K type thermocouples
at 25°C. For a type J output in this temperature range the TC is
51.70 mV/°C, while for a type K it is 40.44 mV/°C. The resulting
gain for the AD594 is 193.4 (10 mV/°C divided by 51.7 mV/°C)
and for the AD595 is 247.3 (10 mV/°C divided by 40.44 mV/°C).
In addition, an absolute accuracy trim induces an input offset to
the output amplifier characteristic of 16 mV for the AD594 and
11 mV for the AD595. This offset arises because the AD594/
compensated signal, the following transfer functions should be
used to determine the actual output voltages:
AD594 output = (Type J Voltage + 16 mV) · 193.4
AD595 output = (Type K Voltage + 11 mV) · 247.3 or conversely:
Type J voltage = (AD594 output/193.4) – 16 mV
Type K voltage = (AD595 output/247.3) – 11 mV
Table I lists the ideal AD594/AD595 output voltages as a func-
tion of Celsius temperature for type J and K ANSI standard
thermocouples, with the package and reference junction at
25°C. As is normally the case, these outputs are subject to cali-
bration, gain and temperature sensitivity errors. Output values
Table I.Output Voltage vs. Thermocouple Temperature (Ambient +25°C, VS = –5 V, +15 V)thermocouples Table I should not be used in conjunction with
European standard thermocouples. Instead the transfer function
given previously and a DIN thermocouple table should be used.
ANSI type K and DIN NICR-NI thermocouples are composed
CONSTANTAN
+5VFigure 1.Basic Connection, Single Supply Operation
SINGLE AND DUAL SUPPLY CONNECTIONSThe AD594/AD595 is a completely self-contained thermocouple
conditioner. Using a single +5 V supply the interconnections
shown in Figure 1 will provide a direct output from a type J
thermocouple (AD594) or type K thermocouple (AD595) mea-
suring from 0°C to +300°C.
Any convenient supply voltage from +5 V to +30 V may be
used, with self-heating errors being minimized at lower supply
levels. In the single supply configuration the +5 V supply con-
nects to Pin 11 with the V– connection at Pin 7 strapped to
power and signal common at Pin 4. The thermocouple wire in-
puts connect to Pins 1 and 14 either directly from the measuring
point or through intervening connections of similar thermo-
couple wire type. When the alarm output at Pin 13 is not used it
should be connected to common or –V. The precalibrated feed-
back network at Pin 8 is tied to the output at Pin 9 to provide a
10 mV/°C nominal temperature transfer characteristic.
AD594/AD595
CONSTANTAN
+5V TO +30V
0V TO –25VFigure 2.Dual Supply Operation
With a negative supply the output can indicate negative tem-
peratures and drive grounded loads or loads returned to positive
voltages. Increasing the positive supply from 5 V to 15 V ex-
tends the output voltage range well beyond the 750°C
temperature limit recommended for type J thermocouples
(AD594) and the 1250°C for type K thermocouples (AD595).
Common-mode voltages on the thermocouple inputs must remain
within the common-mode range of the AD594/AD595, with a
return path provided for the bias currents. If the thermocouple
is not remotely grounded, then the dotted line connections in
Figures 1 and 2 are recommended. A resistor may be needed in
this connection to assure that common-mode voltages induced
in the thermocouple loop are not converted to normal mode.
THERMOCOUPLE CONNECTIONSThe isothermal terminating connections of a pair of thermo-
couple wires forms an effective reference junction. This junction
must be kept at the same temperature as the AD594/AD595 for
the internal cold junction compensation to be effective.
A method that provides for thermal equilibrium is the printed
circuit board connection layout illustrated in Figure 3.
IRON
(CHROMEL)
CONSTANTAN
(ALUMEL)+C–C–IN
–ALMCOMP
COMMONV–VOUTV+Figure 3.PCB Connections
Here the AD594/AD595 package temperature and circuit board
The printed circuit board layout shown also provides for place-
ment of optional alarm load resistors, recalibration resistors and
a compensation capacitor to limit bandwidth.
To ensure secure bonding the thermocouple wire should be
cleaned to remove oxidation prior to soldering. Noncorrosive
rosin flux is effective with iron, constantan, chromel and alumel
and the following solders: 95% tin-5% antimony, 95% tin-5%
silver or 90% tin-10% lead.
FUNCTIONAL DESCRIPTIONThe AD594 behaves like two differential amplifiers. The out-
puts are summed and used to control a high gain amplifier, as
shown in Figure 4.
+IN+C+TCOM–T–CV–
–IN–ALM+ALMV+COMPVOFBFigure 4.AD594/AD595 Block Diagram
In normal operation the main amplifier output, at Pin 9, is con-
nected to the feedback network, at Pin 8. Thermocouple signals
applied to the floating input stage, at Pins 1 and 14, are ampli-
fied by gain G of the differential amplifier and are then further
amplified by gain A in the main amplifier. The output of the
main amplifier is fed back to a second differential stage in an in-
verting connection. The feedback signal is amplified by this
stage and is also applied to the main amplifier input through a
summing circuit. Because of the inversion, the amplifier causes
the feedback to be driven to reduce this difference signal to a
small value. The two differential amplifiers are made to match
and have identical gains, G. As a result, the feedback signal that
must be applied to the right-hand differential amplifier will pre-
cisely match the thermocouple input signal when the difference
signal has been reduced to zero. The feedback network is trim-
med so that the effective gain to the output, at Pins 8 and 9, re-
sults in a voltage of 10 mV/°C of thermocouple excitation.
In addition to the feedback signal, a cold junction compensation
voltage is applied to the right-hand differential amplifier. The
compensation is a differential voltage proportional to the Celsius
temperature of the AD594/AD595. This signal disturbs the dif-
ferential input so that the amplifier output must adjust to restore
the input to equal the applied thermocouple voltage.
The compensation is applied through the gain scaling resistors
so that its effect on the main output is also 10 mV/°C. As a
result, the compensation voltage adds to the effect of the ther-
mocouple voltage a signal directly proportional to the difference
between 0°C and the AD594/AD595 temperature. If the thermo-
The AD594/AD595 also includes an input open circuit detector
that switches on an alarm transistor. This transistor is actually a
current-limited output buffer, but can be used up to the limit as
a switch transistor for either pull-up or pull-down operation of
external alarms.
The ice point compensation network has voltages available with
positive and negative temperature coefficients. These voltages
may be used with external resistors to modify the ice point com-
pensation and recalibrate the AD594/AD595 as described in the
next column.
The feedback resistor is separately pinned out so that its value
can be padded with a series resistor, or replaced with an external
resistor between Pins 5 and 9. External availability of the feedback
resistor allows gain to be adjusted, and also permits the AD594/
AD595 to operate in a switching mode for setpoint operation.
CAUTIONS:The temperature compensation terminals (+C and –C) at Pins 2
and 6 are provided to supply small calibration currents only. The
AD594/AD595 may be permanently damaged if they are
grounded or connected to a low impedance.
The AD594/AD595 is internally frequency compensated for feed-
back ratios (corresponding to normal signal gain) of 75 or more.
If a lower gain is desired, additional frequency compensation
should be added in the form of a 300 pF capacitor from Pin 10
to the output at Pin 9. As shown in Figure 5 an additional 0.01 mF
capacitor between Pins 10 and 11 is recommended.
AD594/
AD595
COMP
300pF
0.01mFFigure 5.Low Gain Frequency Compensation
RECALIBRATION PRINCIPLES AND LIMITATIONSThe ice point compensation network of the AD594/AD595
produces a differential signal which is zero at 0°C and corre-
sponds to the output of an ice referenced thermocouple at the
temperature of the chip. The positive TC output of the circuit is
proportional to Kelvin temperature and appears as a voltage at
+T. It is possible to decrease this signal by loading it with a
resistor from +T to COM, or increase it with a pull-up resistor
from +T to the larger positive TC voltage at +C. Note that
adjustments to +T should be made by measuring the voltage which
tracks it at –T. To avoid destabilizing the feedback amplifier the
measuring instrument should be isolated by a few thousand
ohms in series with the lead connected to –T.
9Figure 6. Decreased Sensitivity Adjustment
this terminal can be produced with a resistor between –C and
–T to balance an increase in +T, or a resistor from –T to COM
to offset a decrease in +T.
If the compensation is adjusted substantially to accommodate a
different thermocouple type, its effect on the final output volt-
age will increase or decrease in proportion. To restore the
nominal output to 10 mV/°C the gain may be adjusted to match
the new compensation and thermocouple input characteristics.
When reducing the compensation the resistance between –T
and COM automatically increases the gain to within 0.5% of the
correct value. If a smaller gain is required, however, the nominal
47 kW internal feedback resistor can be paralleled or replaced
with an external resistor.
Fine calibration adjustments will require temperature response
measurements of individual devices to assure accuracy. Major
reconfigurations for other thermocouple types can be achieved
without seriously compromising initial calibration accuracy, so
long as the procedure is done at a fixed temperature using the
factory calibration as a reference. It should be noted that inter-
mediate recalibration conditions may require the use of a
negative supply.
EXAMPLE: TYPE E RECALIBRATION—AD594/AD595Both the AD594 and AD595 can be configured to condition the
output of a type E (chromel-constantan) thermocouple. Tem-
perature characteristics of type E thermocouples differ less from
type J, than from type K, therefore the AD594 is preferred for
recalibration.
While maintaining the device at a constant temperature follow
the recalibration steps given here. First, measure the device
temperature by tying both inputs to common (or a selected
common-mode potential) and connecting FB to VO. The AD594
is now in the stand alone Celsius thermometer mode. For this
example assume the ambient is 24°C and the initial output VO
is 240 mV. Check the output at VO to verify that it corresponds
to the temperature of the device.
Next, measure the voltage –T at Pin 5 with a high impedance
DVM (capacitance should be isolated by a few thousand ohms
of resistance at the measured terminals). At 24°C the –T voltage
will be about 8.3 mV. To adjust the compensation of an AD594
to a type E thermocouple a resistor, R1, should be connected
between +T and +C, Pins 2 and 3, to raise the voltage at –T by
the ratio of thermocouple sensitivities. The ratio for converting a
type J device to a type E characteristic is:
r (AD594) =(60.9 mV/°C)/(51.7 mV/°C)= 1.18
Thus, multiply the initial voltage measured at –T by r and ex-
perimentally determine the R1 value required to raise –T to that
level. For the example the new –T voltage should be about 9.8 mV.
The resistance value should be approximately 1.8 kW.
The zero differential point must now be shifted back to 0°C.
This is accomplished by multiplying the original output voltage
VO by r and adjusting the measured output voltage to this value
by experimentally adding a resistor, R2, between –C and –T,
Pins 5 and 6. The target output value in this case should be
about 283 mV. The resistance value of R2 should be approxi-
mately 240 kW.
AD594/AD595of R3 should be approximately 280 kW. The final connection
diagram is shown in Figure 7. An approximate verification of
the effectiveness of recalibration is to measure the differential
gain to the output. For type E it should be 164.2.
Figure 7.Type E Recalibration
When implementing a similar recalibration procedure for the
AD595 the values for R1, R2, R3 and r will be approximately
650 W, 84 kW, 93 kW and 1.51, respectively. Power consump-
tion will increase by about 50% when using the AD595 with
type E inputs.
Note that during this procedure it is crucial to maintain the
AD594/AD595 at a stable temperature because it is used as the
temperature reference. Contact with fingers or any tools not at
ambient temperature will quickly produce errors. Radiational
heating from a change in lighting or approach of a soldering iron
must also be guarded against.
USING TYPE T THERMOCOUPLES WITH THE AD595Because of the similarity of thermal EMFs in the 0°C to +50°C
range between type K and type T thermocouples, the AD595
can be directly used with both types of inputs. Within this ambi-
ent temperature range the AD595 should exhibit no more than
an additional 0.2°C output calibration error when used with
type T inputs. The error arises because the ice point compensa-
tor is trimmed to type K characteristics at 25°C. To calculate
the AD595 output values over the recommended –200°C to
+350°C range for type T thermocouples, simply use the ANSI
thermocouple voltages referred to 0°C and the output equation
given on page 2 for the AD595. Because of the relatively large
nonlinearities associated with type T thermocouples the output
will deviate widely from the nominal 10 mV/°C. However, cold
junction compensation over the rated 0°C to +50°C ambient
will remain accurate.
STABILITY OVER TEMPERATUREEach AD594/AD595 is tested for error over temperature with
the measuring thermocouple at 0°C. The combined effects of
cold junction compensation error, amplifier offset drift and gain
error determine the stability of the AD594/AD595 output over
the rated ambient temperature range. Figure 8 shows an AD594/
AD595 drift error envelope. The slope of this figure has units
of °C/°C.
THERMAL ENVIRONMENT EFFECTSThe inherent low power dissipation of the AD594/AD595 and
the low thermal resistance of the package make self-heating
errors almost negligible. For example, in still air the chip to am-
bient thermal resistance is about 80°C/watt (for the D package).
At the nominal dissipation of 800 mW the self-heating in free air
is less than 0.065°C. Submerged in fluorinert liquid (unstirred)
the thermal resistance is about 40°C/watt, resulting in a self-
heating error of about 0.032°C.
SETPOINT CONTROLLERThe AD594/AD595 can readily be connected as a setpoint
controller as shown in Figure 9.
CONSTANTAN
HEATER
20MV
(OPTIONAL)
FOR
HYSTERESIS
SETPOINT
VOLTAGE
INPUT
TEMPERATURE
CONTROLLED
REGION
LOW = > T < SETPOINTFigure 9. Setpoint Controller
The thermocouple is used to sense the unknown temperature
and provide a thermal EMF to the input of the AD594/AD595.
The signal is cold junction compensated, amplified to 10 mV/°C
and compared to an external setpoint voltage applied by the
user to the feedback at Pin 8. Table I lists the correspondence
between setpoint voltage and temperature, accounting for the
nonlinearity of the measurement thermocouple. If the setpoint
temperature range is within the operating range (–55°C to
+125°C) of the AD594/AD595, the chip can be used as the
transducer for the circuit by shorting the inputs together and
utilizing the nominal calibration of 10 mV/°C. This is the centi-
grade thermometer configuration as shown in Figure 13.
In operation if the setpoint voltage is above the voltage corre-
sponding to the temperature being measured the output swings
low to approximately zero volts. Conversely, when the tempera-
ture rises above the setpoint voltage the output switches to
the positive limit of about 4 volts with a +5 V supply. Figure
9 shows the setpoint comparator configuration complete with a
heater element driver circuit being controlled by the AD594/
AD595 toggled output. Hysteresis can be introduced by inject-
ing a current into the positive input of the feedback amplifier
when the output is toggled high. With an AD594 about 200 nA
into the +T terminal provides 1°C of hysteresis. When using a
single 5 V supply with an AD594, a 20 MW resistor from VO to
+T will supply the 200 nA of current when the output is forced
high (about 4 V). To widen the hysteresis band decrease the
resistance connected from VO to +T.
