Multisensor SFH 7773 – crosstalk-free and flexible

The combined proximity and ambient light sensor SFH 7773 of OSRAM Opto Semiconductors is characterized by straightforward application and adaptability to different designs. It works for a wide range of applications without producing crosstalk. Moreover, its detection range can be set and the sensitivity can be adjusted to match the transparency of, for example, Smartphone covers.

Crosstalk is produced by reflections of the emitted light from the top and bottom surface of the cover glass.

Source: OSRAM

The digital Sensor SFH 7773 detects objects as far as 15 cm, and it automatically adjusts the brightness of the display to the ambient light. The device comprises two detectors, for the proximity and the ambient light signal, and an emitter for the proximity function.

Crosstalk-free operation
A proximity sensor emits infrared light pulses which the targeted object reflects back to the detector. However, the sensor receives reflections from the cover glass above the component, too (Fig. 1). These crosstalk signals may significantly exceed the wanted signal from weakly reflecting objects. Designers often intercept crosstalk with an optical barrier between emitter and detector. Alternatively the emitter is placed in a separate package at a distance which prevents crosstalk signals to reach the detector.

Additionally, the thickness of the cover glass and the distance between sensor and cover have an influence on whether crosstalk signals actually reach the detector. The concept of the SFH 7773 is based on exactly that condition: its design does not allow crosstalk to occur with many configurations. Therefore, emitter and detector are placed far apart in the package, and they feature a special emission pattern. Fig. 2 shows the crosstalk-free operating range of the sensor as a function of the distance and the thickness of the cover glass. With increasing distance between sensor and package, the cover glass must be thinner. Or, looking at it the other way around, the thicker the window, the nearer the sensor should be. Depending on its characteristics, tinted glass may reduce the crosstalk-free range. In this case the functionality of the sensor must be checked for all designs. Furthermore, it must be taken into account that with increasing sensitivity of the sensor the crosstalk effect will also increase. Hence the choice of the switching threshold and integration time influences the behavior of the sensor with regard to crosstalk.

The SFH 7773 also works beyond the range shown in Fig. 2, of course. Crosstalk may occur there which, among other causes, may depend on the type of cover.  To ensure reliable functionality for such configurations, OSRAM recommends the installation of an optical barrier.

Setting the sensitivity
The SFH 7773 facilitates the adjustment of the sensitivity of the proximity and ambient light detectors via the I²C bus. That way the detection range of the proximity sensor can be altered and the ambient light sensor can be adapted to the transparency of the respective cover. To change the sensitivity, different integration times are selected for both detectors. In the original state of the IC, these are 300 ms for the proximity signal and 100 ms for the ambient light signal. The setting is carried out via the registers of the I²C bus. The SFH 7773 data sheet and the application note provide information on the registers and the available values for integration times.

The detection range of the proximity sensor not only depends on the sensitivity of the detector, but also on the strength of the emitter current. For instance, at a forward current of 200 mA and an integration time of 750 ms, objects as far as 15 cm away can be detected. For the integration time, ten stages between 100 and 2500 ms are available. 

SFH 7773 offers variable integration times for the ambient light signal, too. The most sensitive setting with 1 s integration time detects a brightness range of between 0.03 and 655 Lux. Thus the sensor works reliably even under a tinted cover glass which permits less than 655 Lux passing to the detector. For manufacturers this option currently gains significance since they increasingly use tinted windows to give the device surface a homogeneous appearance. The shortest integration time of 10 ms serves applications in a very bright environment with 3 up to approximately 65.500 Lux. Nine different settings between 1 s and 10 ms can be chosen from. Thanks to variable integration times, the SFH 7773 is very versatile. Manufacturers can now apply one design to various device models and different applications of the proximity function.

See also the related Press Release.