Infrared Thermometer Circuit Design for Battery Powered Units

A non contact thermometer measures a tiny voltage from a thermopile and turns it into a temperature that the user trusts. The infrared thermometer circuit design problem is dominated by microvolt level signals, ambient compensation and a battery that has to last between charges.

The Signal Chain

The thermopile produces a small voltage proportional to the difference between the target and the sensor body, with a sensitivity of tens of microvolts per degree. That voltage has to be amplified before it can be digitised accurately.

The chain is short: sensor, amplifier, analogue to digital converter and reference. Because the signal is so small, every element in that chain contributes to the error budget and to the drift that appears over temperature.

Choosing the Amplifier

A chopper stabilised op amp is the usual first choice, because its input offset voltage and offset drift are measured in microvolts. Those two parameters set the floor of the temperature accuracy and cannot be calibrated away over a wide ambient range.

Rail to rail input and output helps with dynamic range on a low supply, and a low quiescent current keeps the analogue stage from dominating the battery budget. The trade is bandwidth, which a measurement channel does not need.

Ambient Measurement and Compensation

The thermopile reports a difference, so the absolute temperature requires a second measurement of the sensor body. A digital temperature sensor with an accuracy of a fraction of a degree is convenient because it needs no calibration and no analogue routing.

Place that temperature sensor in the same thermal environment as the thermopile case. If the two are separated by a heat source or by a copper path to the outside, the compensation becomes wrong exactly when the unit is warming up in the hand.

Converter and Reference

The converter has to resolve a signal that spans only a few millivolts after gain. A sigma delta converter with a stable reference is easier to use here than a fast successive approximation device, because it averages the signal and rejects the mains frequency naturally.

The reference is part of the measurement. A reference that drifts with temperature adds an error that is indistinguishable from a real reading, so the low dropout regulator that feeds the analogue supply and the reference should be the quietest rail on the board.

The Power Architecture

Handheld units are usually powered by a single cell or a pair of cells, so a boost stage lifts the battery to a working rail and a linear regulator provides a clean analogue supply. The two rails serve different purposes and should be treated separately.

The boost converter handles the load transients and the display current, while the low dropout regulator feeds the sensor and the converter. Keep the switching node away from the analogue input, and place the input capacitor of the regulator next to the pin.

Low Quiescent Current Design

The quiescent current of every rail adds up. A boost converter, a regulator, an amplifier and a sensor that draw a few microamps each become the difference between a month and a year of standby life.

Choose converters with a low quiescent current and a shutdown current in the nanoamp range, and put the display and the radio behind a switch. Wake up sources should be limited to the button and the real time clock.

Display and User Interface

The display is often the largest load. A segmented display can be driven directly and costs little power, while a backlit graphical display needs a boost rail and a driver with dimming.

Button response has to be immediate when the unit wakes, which argues for an interrupt from the button rather than a periodic poll. The first measurement should be ready while the user is still aiming the device.

Optics and Sensor Placement

The thermopile has a field of view defined by its window, and its accuracy depends on that window staying clean and at the ambient temperature of the body. The mechanical design of the aperture is therefore part of the electrical design.

Keep heat sources such as the regulator and the display away from the sensor, and avoid copper planes that would conduct heat from the processor into the sensor body. A small thermal break in the copper beneath the sensor is worth the effort.

Calibration and Traceability

Each unit is usually calibrated against a reference at one or two points, and the coefficients are stored in non volatile memory. Calibration compensates the gain and offset of the chain, but not the drift of the sensor itself.

Records should link the serial number to the calibration date and the reference used. A medical or industrial product needs that traceability as much as it needs the accuracy.

Electromagnetic and Layout Notes

The analogue input is the most sensitive node, so keep it short, guard it with ground and keep it away from the switching supply. Any high current return path that runs beneath the amplifier injects a small voltage that looks exactly like a signal.

On a mixed signal board the analogue and digital returns should be separated locally and joined at one point, as described in our mixed signal layout notes and in the layout guidance for DC to DC converters.

Verification

Test the unit against a black body reference across the full ambient range, and record the error at several target temperatures rather than at one. The shape of that error curve is what shows whether the compensation is working.

Then test the battery life with a realistic duty cycle. A measurement channel that is accurate but that flattens the cell in a week is a design failure, and the fix belongs in the current budget and in the sleep strategy.

Handling and Field Use

The unit is used at different ambient temperatures and often brought from a cold room into a warm one, so the sensor window fogs. The mechanical design and the measurement timing both have to allow for that.

A short warm up before the reading is taken, and a note in the instructions, is usually enough. The alternative is a heater, which adds current and complexity to a battery powered product.

Standards and Documentation

Where the product is used for a clinical or an industrial measurement, the accuracy claim has to be supported by a test method and by records. The circuit design then follows from the required accuracy rather than from the available parts.

The documentation should state the field of view, the emissivity assumption, the ambient range and the calibration interval. A number quoted without those conditions is not a specification.

Power Budget Worksheet

A power budget is easier to keep than to reconstruct. List every rail, its quiescent current, its load current in each mode and its efficiency, then sum the result for the standby, measurement and display cases.

The worksheet makes the design decision visible: which rail to shut down, which converter to replace and where the display dominates. It is also the document that predicts the battery life the product will achieve.

FAQ

Why not digitise the thermopile directly? The signal is too small for the resolution of a practical converter. Gain in the analogue domain costs less accuracy than gain in the digital domain, and it removes the offset error of the converter from the result.

How often does the unit need calibration? The stored coefficients do not change, but the sensor and the optics do. Most products are verified annually against a reference, and the interval should follow the accuracy the application requires.

Does the emissivity setting matter? It does for surfaces that are not close to a black body. Measuring a shiny metal part without correcting for its emissivity gives a reading that is well below the true temperature.