AD595CDZ

Monolithic Thermocouple Amplifiers with Cold Junction Compensation

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: 15 V Low Power: <1 mW typical Thermocouple Failure Alarm Laser Wafer Trimmed to 1C Calibration Accuracy Setpoint Mode Operation Self-Contained Celsius Thermometer Operation High Impedance Differential Input Side-Brazed DIP or Low Cost Cerdip

PRODUCT DESCRIPTION The 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 to be used as a linear amplifier-compensator 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 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, temperatures below 0°C can be measured. To minimize self-heating, an unloaded AD594/AD595 will typically operate with a total supply current 160 µA, but is also capable of delivering in excess 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 calibration for both thermocouple and thermometer applications.

The AD594/AD595 is available in two performance grades. The C and the A versions have calibration accuracies ± 1°C and ± 3°C, respectively. Both are designed to be used from to +50°C, and are available in 14-pin, hermetically sealed, sidebrazed ceramic DIPs as well as low cost cerdip packages.

PRODUCT HIGHLIGHTS 1. The AD594/AD595 provides cold junction compensation,

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 V to dual supplies spanning 30 V.

5. Differential input rejects common-mode noise voltage on the thermocouple leads.

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.

AD594/AD595­SPECIFICATIONS +25C and 5 V, Type J (AD594), Type K (AD595) Thermocouple, unless otherwise noted)

ABSOLUTE MAXIMUM RATING +VS to ­VS Common-Mode Input Voltage Differential Input Voltage Alarm Voltages +ALM ­ALM Operating Temperature Range Output Short Circuit to Common

AMPLIFIER CHARACTERISTICS Closed Loop Gain3 Input Offset Voltage

Input Bias Current Differential Input Range Common-Mode Range Common-Mode Sensitivity ­ RTO Power Supply Sensitivity ­ RTO Output Voltage Range

Dual Supply Single Supply Usable Output 3 dB Bandwidth

ALARM CHARACTERISTICS VCE(SAT) 2 mA Leakage Current Operating Voltage at ­ ALM Short Circuit Current

POWER REQUIREMENTS Specified Performance Operating5 Quiescent Current (No Load) +VS ­VS

NOTES 1Calibrated for minimum error at +25°C using a thermocouple sensitivity of 51.7 µV/°C. Since a J type thermocouple deviates from this straight line approximation, the AD594 will normally read 3.1 mV when the measuring junction at 0°C. The AD595 will similarly read 2Defined 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. 4Current Sink Capability in single supply configuration is limited to current drawn to ground through 50 k resistor at output voltages below V. 5­VS 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.

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

to match the transfer characteristic of J and K type thermocouples at 25°C. For a type J output in this temperature range the is 51.70 µV/°C, while for a type is 40.44 µV/°C. The resulting gain for the 193.4 (10 mV/°C divided by 51.7 µV/°C) and for the 247.3 (10 mV/°C divided by 40.44 µV/°C). In addition, an absolute accuracy trim induces an input offset to the output amplifier characteristic 16 µV for the AD594 and 11 µV for the AD595. This offset arises because the AD595 is trimmed for 250 mV output while applying a 25°C thermocouple input.

Because a thermocouple output voltage is nonlinear with respect

compensated signal, the following transfer functions should be used to determine the actual output voltages:

AD594 output = (Type J Voltage + 16 µV) 193.4 AD595 output = (Type K Voltage + 11 µV) 247.3 or conversely:

Table I lists the ideal AD594/AD595 output voltages as a function of Celsius temperature for type J and K ANSI standard thermocouples, with the package and reference junction As is normally the case, these outputs are subject to calibration, gain and temperature sensitivity errors. Output values for intermediate temperatures can be interpolated, or calculated using the output equations and ANSI thermocouple voltage tables referred to zero degrees Celsius. Due to a slight variation in alloy content between ANSI type J and DIN FE-CUNI

Table I. Output Voltage vs. Thermocouple Temperature (Ambient +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

Figure 1. Basic Connection, Single Supply Operation

of identical alloys and exhibit similar behavior. The upper temperature limits in Table I are those recommended for type J and type K thermocouples by the majority of vendors.

SINGLE AND DUAL SUPPLY CONNECTIONS The 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 to +300°C.

Any convenient supply voltage from +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.

By using a wider ranging dual supply, as shown in Figure 2, the AD594/AD595 can be interfaced to thermocouples measuring both negative and extended positive temperatures.