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Analog Devices CN0354

Analog Devices CN0354 Low Power Multichannel Thermocouple Measurement System

The circuit shown below is a flexible, 4-channel, low power thermocouple measurement circuit with an overall power consumption of less than 8 mW. The circuit has a multiplexed front end, followed by an instrumentation amplifier that performs cold junction compensation (0°C to 50°C) and converts the thermocouple output to a voltage with a precise scale factor of 5 mV/°C. The error is less than 2°C, over a measurement range of −25°C to +400°C, and is primarily due to the thermocouple nonlinearity. A nonlinearity correction algorithim reduces the error to less than 0.5°C over a 900°C measurement range. Noise free resolution is less than 0.1°C.

The signal is then digitized by a 24-bit Σ-Δ ADC, and the digital value is provided on an SPI serial interface. With the PMOD form factor for rapid prototyping, the design requires minimal PC board area and is ideal for applications that require precise thermocouple temperature measurements.

Benefits

The humidity sensor output is digitized by the AD7745, 24-bit, Σ-Δ capacitance-to-digital converter (CDC). A 2-wire, I2C-compatible interface allows access to the internal configuration registers as well as to the data conversions.

 The AD8615 rail-to-rail amplifier with very low offset voltage (65 μV) and wide signal bandwidth (>20 MHz) acts as a unity-gain buffer and provides the appropriate drive signal to the sensor.

Parts used

  • AD7787 low power, complete analog front end for low frequency measurement applications.
  • AD8495 precision instrumentation amplifier with thermocouple cold junction compensators on an integrated circuit. 
  • ADG1609 monolithic CMOS analog multiplexers comprising eight single channels and four differential channels, respectively. 
  • ADM8829 charge-pump voltage inverter which may be used to generate a negative supply from a positive input. Input voltages ranging from +1.5 V to +5.5 V can be inverted into a negative –1.5 V to –5.5 V output supply. 
  • ADR3412 low-cost, low-power, high precision voltage references, featuring ± 0.1% initial accuracy, low operating current and low output noise in a small SOT23 package.
  • REF194 precision band gap voltage references use a patented temperature drift curvature correction circuit and laser trimming of highly stable, thin-film resistors

Test Results

The AD8495 has a maximum temperature error due to the thermocouple nonlinearity of ±2°C from −25°C to +400°C, for a reference junction temperature of 0°C to 50°C. Wider temperature ranges or better accuracy requires a linearity correction algorithm that can be implemented in the software.

Block Diagram