Temperature measurement IC digitises with 0.1°C conformity

Article By : Graham Prophet

The LTC2986 adds three new operating modes and reduces the number of analogue inputs from 20 channels to 10 channels.

A high performance digital temperature measurement IC, the LTC2986 directly digitises any combination of thermocouples, RTDs, thermistors and external diodes with 0.1°C conformity and 0.001°C resolution.

The LTC2986 builds on the (pin-compatible) LTC2983 and LTC2984 by adding three new operating modes and reducing the number of analogue inputs from 20 channels to 10 channels. The new operating modes provide better support for external overvoltage protection resistors that are shared between multiple sensor types, powered temperature sensors with analogue outputs, and other non-temperature related sensors such as pressure or other voltage output sensors.

The IC’s high performance analogue front end combines low noise and low offset buffered ADCs with all the necessary excitation and control circuits for each sensor. Measurements are performed under the control of a digital engine, combining all the algorithms and linearisation required for each. The device precisely measures absolute microvolt level signals from thermocouples and ratiometric resistance measurements from RTDs and thermistors, performs the linearisation and outputs the results in °C or °F. Up to 10 analogue inputs are available, allowing support for up to nine thermocouples, four RTDs, four thermistors and/or 10 diodes. The SPI interface works with virtually any digital system and a comprehensive software support system with drop-down menus allows easy customisation of the LTC2986.

The LTC2986 interfaces with a wide variety of temperature sensors, including type B, E, J, K, N, S, R, T thermocouples, 2-, 3- or 4-wire RTDs, 2.25 kΩ to 30 kΩ thermistors and temperature sensing diodes. The LTC2986 works with ground-referenced sensors without the need of amplifiers, negative supplies or level shift circuitry. Signals are simultaneously digitised with three, high accuracy, 24-bit Δ-Σ ADCs using an internal 15ppm/°C reference. Automatic thermocouple cold junction compensation is accomplished using any type of external sensor. Included on the chip are linearisation algorithms for all common sensor types. Custom sensors can be linearised with custom coefficients, programmed and stored in the chip.

Onboard EEPROM (LTC2986-1) is used to store user configuration data and custom sensor coefficients, eliminating any IC or sensor programming by a host processor. Dual programmable excitation current sources feature current reversal and current ranging to improve accuracy and reduce noise. To ensure resistive measurements are accurate, current reversal eliminates thermocouple effects in the resistive sensor. Sensor-specific fault detection alerts the user of short-circuits, open-circuits, over-temperature, under-temperature and ADC over-ranging.

The LTC2986 is offered in commercial, industrial and automotive versions; in a 7mm x 7mm LQFP-48 package, pricing starts at ₹1,110.44 ($16.56) per 1,000 pieces.

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