FIRE SAFETY
Addressing the new ChAllenge ADI has created a technology to help address these issues: the ADPD188BI. It directly integrates two LEDs (blue and infrared), a photodiode, and an analog front end. Digital output over I²C or SPI enables a connection to a microcontroller. A block diagram of the ADPD188BI is shown in Figure 3. As can be seen in the figure, the complete signal chain is realised in a single 5mm by 3.8mm chip. The ADPD188BI works by emitting a short
LED pulse of a few microseconds. Some of this light is scattered by the smoke particles back onto the photodiodes (see the cross- section of the ADPD188BI in Figure 4). The analogue front end (AFE) includes the transimpedance amplifier, band-pass filter, integrator, ADC, LED drivers, and digital control (see the middle of Figure 3). There are many options inside the AFE to enable optimisation for different applications and use cases. The AFE also provides the ability to reject ambient light, such as from lamps or solar radiation at levels up to 80dB. This principle provides many significant
advantages. The short distance between the LED and PD results in much more efficient use of light, which reduces the power dissipation of the system for a required sensitivity and results in a longer battery lifetime. Two different LED colours are included in the device. The amount of light scattered by the particles is a function
Figure 4. Cross- section of the ADPD188BI.
of wavelength. This provides limited use for separating the hamburger and flaming polyurethane smokes, but can be used to distinguish between relatively small smoke particles (100nm to 300nm diameter) and much larger smoldering plastic or steam
particles (10μm diameter). The highly configurable AFE inside the ADPD188BI provides for a very high dynamic range that is software configurable and can be adjusted on the fly. The SNR of the system can also be easily adjusted to optimise for power or performance on the fly. One example would be to dynamically increase the sample rate or SNR when smoke has been detected to more accurately differentiate between a nuisance source or a real fire. The integration of the system also enables Analog Devices to
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calibrate the loop response (LED driver LED PD AFE) of the parts and burn calibration coefficients into the AFE to limit the part-to- part variation to better than ±10 per cent, which reduces or eliminates the need for expensive and time-consuming sensitivity calibrations in smoke tunnels. The ADPD188BI provides the features and capabilities to increase the performance of smoke detectors to more accurately separate nuisance sources from fire events. As well as performance, there are further advantages: the integration of LEDs means separate LED sourcing and stocking is eliminated, and the small form factor allows for integrated smoke detection across intelligent building components.
Analog Devices
www.analog.com
InDustrIAl ComplIAnCe | mArCH/AprIl 2022 19
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