ADI's Precision Thermocouple Amplifiers with Integrated Cold Junction Compensation

Thermocouple systems provide accurate and reliable temperature measurements and control, are cost-effective, and can measure a wide range of temperatures. However, to achieve good measurement performance cold junction compensation and a high gain accuracy precision amplifier are required. Analog Devices’ AD8494, AD8495, AD8496 and AD8497 thermocouple amplifiers address this issue by featuring on-chip cold junction compensation that automatically adjusts the thermocouple output for changes in ambient temperature. This feature greatly simplifies the design process and reduces system cost relative to similar discrete solutions.

“We’re using ADI’s thermocouple amplifiers in designs for ultra-pure heating controllers for industrial applications,” said Bob Woskow, president, Robert Marshall and Associates, an Encino, California-based embedded microcontroller design and development consultancy. “ADI’s amplifiers are smaller, complete with internal cold junction compensation and thus are quite simple to use with few or no additional parts needed on the printed circuit board. They will do everything we need a thermocouple amplifier to do.”

The four new precision thermocouple amplifiers each convert the small voltages generated by a thermocouple junction into a 5 mV/°C analog signal that can easily be read by an ADC (analog-to-digital converter) or microcontroller. The integrated cold junction compensation ensures accurate measurement over a wide range of ambient temperatures while the high common-mode rejection (0.1°C/V) allows the AD849x to reject common-mode noise that may be associated with long lead thermocouples. The AD849x thermocouple amplifiers operate off a wide variety of voltages, from single 2.7-V to dual ±18-V supplies and are very power efficient, consuming only 180 uA quiescent supply current. In single-supply operation the amplifiers can directly interface to lower-supply ADCs while higher, dual-supply voltages can be used for industrial systems that require a wide common mode input range. With a single 5-V supply, the AD849x amplifiers can cover nearly 1,000-degrees of a thermocouple’s temperature range. A reference pin allows for level shifting of the 0°C output so that negative temperatures can easily be measured, even on a single supply.

“Temperature is the second most measured parameter. Systems using ADI’s linear products provide rugged, repeatable temperature sensing with simplified design,” said Steve Sockolov, director, Precision Signal Conditioning Group, Analog Devices. “Integrated cold junction compensation greatly simplifies thermocouple system design for use in all applications including medical dialysis machines, blood analyzers, process control and instrumentation.”

Thermocouple Amplifier Key Features and Benefits

* Integrated cold junction compensation
* 5-mV/°C output with single-supply operation down to 2.7 V and rail-to-rail output
* High-impedance differential input
* Reference pin allows offset adjustment
* Wide power-supply range: 2.7 V to ±18 V
* High Gain Accuracy – 0.1%
* Low power: 180 µA quiescent supply current
* Low cost: less than US$1.00, in volume
* Lead-free MSOP-8 package