Temperature-measurement scheme uses IR sensor and sigma-delta ADC

-April 03, 2003

Many noncontact temperature-measurement systems use infrared sensors, such as thermopiles, which can detect small amounts of heat radiation. Biomedical thermometers that measure the temperature of an ear or a temple use noncontact temperature measurement, as do automotive-HVAC systems that adjust temperature zones based on the body temperature of passengers. Household appliances and industrial processes can also benefit from the use of noncontact temperature measurement. Infrared thermometers can measure objects that move, rotate, or vibrate, measuring temperature levels at which contact probes either would not work or would have a shortened operating life. Infrared measurements do not damage or contaminate the surface of the item being measured. Thermal conductivity of the object being measured presents no problem, as would be the case with a contact temperature-measurement device. The circuit in Figure 1 provides a design for a high-resolution digital thermometer that uses a thermopile sensor and a sigma-delta ADC. The design provides high resolution and response times of approximately 1 msec, and it eliminates the need for high-performance, low-noise signal conditioning before the ADC.

The high-accuracy, noncontact digital temperature measurement system uses the MLX90247D thermopile from Melexis (www.melexis.com) and the AD7719 high-resolution, sigma-delta ADC from Analog Devices (www.analog.com). The AD7719 provides differential inputs and a programmable-gain amplifier; thus, you can connect it directly to the sensor, allowing the temperature-measurement system to provide high accuracy without the need for precision signal-conditioning components preceding the ADC. The MLX90247D sensor comprises a thin, micromachined membrane embedded with semiconductor thermocouple junctions. The Seebeck-coefficient thermocouples generate a dc voltage in response to the temperature differential generated between the hot and the cold junctions. The low thermal conductivity of the membrane allows absorbed heat to cause a higher temperature increase at the center of the membrane than at the edge, thus creating a temperature difference that is converted to an electric potential by the thermoelectric effect in the thermopile junctions. The MLX90247D also contains a thermistor, allowing you to configure a temperature-compensated system in relative-measurement mode.

The AD7719, a dual-channel, simultaneously converting ADC with an internal programmable-gain amplifier is an ideal ADC when you use it with the MLX90247D sensor in temperature-measurement applications. The main channel is 24 bits wide, and you can configure it to accept analog inputs of 20 mV to 2.56V at update rates of 5 to 105 Hz. The auxiliary channel contains a 16-bit ADC and accepts full-scale analog inputs of 1.25 or 2.5V with an update rate equal to that of the main channel. The AD7719 accepts signals directly from the sensor; the internal programmable-gain amplifier eliminates the need for high-accuracy, low-noise external-signal conditioning. The AD7719 simultaneously converts both the thermopile and the thermistor sensor outputs. The main channel with its programmable-gain amplifier monitors the thermopile, and the auxiliary channel monitors the thermistor. You can use on-chip chopping and calibration schemes in optimizing the design. The AD7719 features a flexible serial interface for accessing the digital data and allows direct interface to all controllers.

The sensitivity of the thermopile is 42 µV/K; thus, it produces an output voltage of 9.78 to 15 mV over the industrial temperature range of –40 to +85°C, an output that the AD7719 can directly measure. The thermistor's impedance ranges from 15.207 kΩ at –40°C to 38.253 kΩ at +85°C with a nominal impedance of 26 kΩ at 25°C. Again, you can directly measure voltages from the thermistor, as Figure 1 indicates. Biomedical thermometers generally have a measurement range of 34 to 42°C. In this range, the thermopile's differential output is 336 µV. Operating the AD7719 in its ±20-mV input range with a 5-Hz update rate allows temperature measurement with a resolution of 0.05°C.

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