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AD595 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer AD595
Beschreibung Monolithic Thermocouple Amplifiers with Cold Junction Compensation
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 8 Seiten
AD595 Datasheet, Funktion
a
Monolithic Thermocouple Amplifiers
with Cold Junction Compensation
AD594/AD595
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/؇C
Built-In Ice Point Compensation
Wide Power Supply Range: +5 V to ؎15 V
Low Power: <1 mW typical
Thermocouple Failure Alarm
Laser Wafer Trimmed to 1؇C Calibration Accuracy
Setpoint Mode Operation
Self-Contained Celsius Thermometer Operation
High Impedance Differential Input
Side-Brazed DIP or Low Cost Cerdip
FUNCTIONAL BLOCK DIAGRAM
–IN –ALM +ALM
14 13 12
AD594/AD595
V+ COMP
11 10
OVERLOAD
DETECT
+A
VO
9
FB
8
GG
ICE
POINT
COMP. –TC
+TC
12 3 4 56 7
+IN +C +T COM –T –C V–
PRODUCT DESCRIPTION
The 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 µA, but is also capable of delivering in excess of
± 5 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 and
± 3°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 HIGHLIGHTS
1. The AD594/AD595 provides cold junction compensation,
amplification, and an output buffer in a single IC package.
2. Compensation, zero, and scale factor are all precalibrated by
laser wafer trimming (LWT) of each IC chip.
3. Flexible pinout provides for operation as a setpoint control-
ler or a stand-alone temperature transducer calibrated in
degrees Celsius.
4. 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.
5. Differential input rejects common-mode noise voltage on the
thermocouple leads.
REV. C
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., 1999






AD595 Datasheet, Funktion
AD594/AD595
of R3 should be approximately 280 k. 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.
1 +IN
COM 4
14 –IN
AD594/
AD595
+T 3
+C 2
9 VO
–C 6
8 FB
–T 5
R1
R2
R3
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 , 84 k, 93 kand 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 AD595
Because 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 TEMPERATURE
Each 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.
+0.6؇C
THERMAL ENVIRONMENT EFFECTS
The 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 µW 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 CONTROLLER
The AD594/AD595 can readily be connected as a setpoint
controller as shown in Figure 9.
HEATER
DRIVER
CONSTANTAN
HEATER (ALUMEL)
14
LOW = > T < SETPOINT
TEMPERATURE
COMPARATOR OUT
+5V
HIGH = > T > SETPOINT
SETPOINT
VOLTAGE
INPUT
13 12 11 10
9
8
AD594/
AD595
OVERLOAD
DETECT
+A
20M
(OPTIONAL)
FOR
HYSTERESIS
GG
ICE
POINT –TC
+TC COMP.
IRON
(CHROMEL)
123 4 567
TEMPERATURE
CONTROLLED
REGION
COMMON
Figure 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 Mresistor 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.
0
–0.6؇C
25؇C
50؇C
TEMPERATURE OF AD594C/AD595C
Figure 8. Drift Error vs. Temperature
–6–
REV. C

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