A Single Chip Temperature Measurement Solution
The Linear Technology LTC2983 is a multi-sensor, high accuracy digital temperature measurement system in a single chip. Its twenty input channels can accommodate a wide range of temperature sensors, including directly connected thermocouples, platinum resistance elements (RTD’s), thermistors and diodes.
The LTC2983 stores the parameters for each input channel as 32 bit values, requiring a total of 20x4 = 80 bytes (accessed at memory locations 0x200 - 0x24F), while a separate 80 byte area (0x10 - 0x5F) is used to return the conversion results - which again are 32 bits. On-board multiplexed 24 bit sigma-delta ADC’s perform the measurements.
On power-up and after all required channel assignments have been made, data can be written into the command status register (at address 0x00) in order to select the channel to read and to initiate a measurement. This same register can then be polled to confirm that the measurement has completed and after that, the resulting 4 bytes of data can then be read back from the appropriate temperature data memory address to obtain the result.
Interfacing to the LTC2983 is straightforward via an SPI bus and temperature readings using the LTC2983 take approximately 200 milliseconds per point.
A Propeller-based temperature measurement system has been developed as seen in the photo below. This PCB incorporates the P8X32 Propeller chip and its support circuitry, the LTC2983, plus a Nokia 5110 LCD display and a microSD card slot to accommodate data logging applications.
As mentioned above, the LTC2983 affords the user great flexibility in terms of the range of different temperature sensors that be connected. In the photo above, a platinum resistance thermometer, a thermistor, two type K thermocouples and a standard analog input are all connected to LTC2983 inputs via the two 12-way connector blocks. The two resistors seen in the photo are each connected to pairs of input channels on the LTC2983. To measure temperature with either an RTD or thermistor, a user-specified excitation current is passed through the series-connected sense resistor and the temperature sensor. The LTC2983 measures the voltage drop across the sensor and converts this to an absolute temperature.