Thermopile Sensor Interface Circuit Design

A thermopile sensor measures infrared radiation from a surface and turns it into a small voltage, typically tens of microvolts per degree of temperature difference. Because the signal is tiny and the sensor also responds to its own temperature, the interface has to solve a low level amplifier problem and a reference problem at the same time.

What the Sensor Produces

A thermopile is a series string of thermocouples on a thin membrane. The hot junctions face the target through a window or a filter, the cold junctions sit on the package body, and the output is proportional to the difference between the two. The voltage is small and has a high source impedance.

The output therefore contains two pieces of information: the radiation from the target and the temperature of the package itself. Without the second, the reading is meaningless, which is why every thermopile module includes a temperature sensor for the cold junction on the same substrate.

The window or filter sets the spectral response. A silicon window passes the near infrared and blocks visible light, a germanium window covers a different band, and a filter may be fitted to match a specific gas absorption line. For an infrared temperature measurement, the filter is what separates a general purpose sensor from a calibrated one.

the field of view and Optical Design

The field of view is the cone from which the sensor collects radiation. It is defined by an aperture or a lens in front of the element, and it is usually quoted as the half angle at which the response falls to half its peak value.

A narrow field of view measures a small spot at a distance, which is what an infrared thermometer needs when the target is small. A wide field of view averages a large area and suits a presence or motion detector. Choosing the wrong one produces a reading that is an average of the target and its surroundings.

The distance to spot ratio follows from the field of view. At a ratio of ten to one, a sensor one metre from the target measures a spot about ten centimetres across. The target must fill that spot, and the surrounding surfaces must not be hotter or colder than the object of interest.

Building the signal conditioning Stage

Signal conditioning starts with a chopper stabilised or low offset operational amplifier in a differential configuration. The offset of an ordinary amplifier drifts with temperature, and a drift of a few microvolts per degree is comparable with the signal from a one degree change in target temperature.

A gain between one hundred and one thousand is typical, followed by a low pass filter with a corner of a few hertz. The corner has to be low enough to reject mains hum and the noise of the amplifier, and the filter should be placed after the first gain stage so that the following stages see a larger signal.

Chopper amplifiers have a small amount of switching artifact at the chopping frequency, so the filter has to reject that as well. A two pole active filter with a corner well below the chopping frequency removes most of it, and the residual appears as a fixed offset that the calibration removes.

The Cold Junction Reference

The package temperature is measured with a thermistor, a silicon bandgap sensor or an integrated digital sensor. Its accuracy matters directly, because an error of one degree in the package temperature is an error of roughly one degree in the indicated target temperature.

Place the reference sensor as close to the thermopile package as the layout allows and couple the two thermally. The reference should see the same temperature as the cold junctions, and a small copper island shared by both is better than a sensor sitting on the far side of the board.

Avoid placing the reference near a heat source. A regulator, a display backlight or a power resistor within a centimetre can raise the local temperature by several degrees and create an offset that appears only after the product has been running for a while.

Reducing the noise floor

Noise floor is set by the amplifier, the bandwidth and the source resistance. Since the source resistance of a thermopile is high, the current noise of the amplifier matters as much as the voltage noise, and a part with a low voltage noise but a high current noise is a poor choice here.

Narrowing the bandwidth is the cheapest improvement. A sensor used for a temperature reading does not need to respond in milliseconds, so a corner of one or two hertz removes a large part of the broadband noise without affecting the measurement.

Screening is the second half of the answer. The high impedance node between the sensor and the amplifier picks up electrostatic fields, and a guard ring driven at the same potential, or a short screened connection, keeps the leakage and the coupling out of the input.

Layout and Thermal Considerations

Keep the sensor and the first stage on a small island with a solid ground plane underneath. The plane provides a thermal mass that slows the local temperature change and a return path that keeps the loop area of the input small.

Do not place the amplifier in the air stream of a fan or next to a vent. A moving air stream produces a fluctuating temperature gradient across the board, and the resulting thermal electromotive force appears as low frequency noise that looks like sensor drift.

Thermocouple effects in the joints between dissimilar metals also matter at these levels. A solder joint between copper and a plated pad generates a few microvolts per degree, so the number of such junctions in the input path should be minimised and they should be kept isothermal.

Calibration and Verification

Calibration uses a black body source at a known temperature and a known emissivity. The instrument is pointed at the source, and the gain and offset terms are fitted from readings at several temperatures spread across the working range.

A single point calibration is enough for a relative measurement but not for an absolute one, because the gain of the amplifier and the sensitivity of the sensor both vary. A two point fit over the range removes the first order errors and is what most instruments ship with.

Verification in the field usually means comparing the reading against a contact thermometer on the same surface. A difference that grows with target temperature points to a gain error, while a constant difference is an offset or an emissivity error. The emissivity of the surface is the most common cause of a reading that is wrong by more than a few degrees, and no amount of circuit work will correct it. The release checks that keep a low level analog front end consistent between units are collected in our PCB design release checklist.

Common Problems and Their Causes

A reading that increases when the product warms up is usually a self heating problem. The regulator or the processor heats the board, the cold junction reference follows, and the indicated target temperature rises with it. Moving the heat source or the sensor is the cure.

A reading that jumps when a light is switched on points to a filter problem. The window passes some visible light, and a bright source within the field of view saturates the amplifier. A filter matched to the sensor and a check of the input range solve it.

A reading that is noisy on some units and not others is often a layout difference. Compare the guard ring, the input trace length and the placement of the first stage. The points we look for on a finished assembly are described in our guide to judging PCB quality, and the wider question of keeping a mixed signal front end quiet is covered in mixed signal board design.

FAQ

Why is my thermopile reading off by several degrees? Emissivity is the usual cause. A shiny metal surface radiates far less than a black one, and the instrument reads the reflected background instead.

Do I need a chopper amplifier? For a target temperature change of a few degrees, yes. An ordinary amplifier offset drifts more than the signal over the operating temperature range.

How close can the sensor be to the target? As close as the field of view allows. The target must fill the spot, so get closer with a narrow field of view or a larger target.