27-02-2013, 02:14 PM
Monolithic Thermocouple Amplifiers with Cold Junction Compensation
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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 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 thermocouple
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.
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 controller
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.
INTERPRETING AD594/AD595 OUTPUT VOLTAGES
To 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/
AD595 is trimmed for a 250 mV output while applying a 25°C
thermocouple input.
SINGLE AND DUAL SUPPLY CONNECTIONS
The 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) measuring
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 connects
to Pin 11 with the V– connection at Pin 7 strapped to
power and signal common at Pin 4. The thermocouple wire inputs
connect to Pins 1 and 14 either directly from the measuring
point or through intervening connections of similar thermocouple
wire type. When the alarm output at Pin 13 is not used it
should be connected to common or –V. The precalibrated feedback
network at Pin 8 is tied to the output at Pin 9 to provide a
10 mV/°C nominal temperature transfer characteristic.
RECALIBRATION PRINCIPLES AND LIMITATIONS
The ice point compensation network of the AD594/AD595
produces a differential signal which is zero at 0°C and corresponds
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.
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 ambient
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 selfheating
error of about 0.032°C.
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 ambient
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 compensator
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.