Circuit Note

CN-0537

Rev. A | Page 5 of 8

When selecting a sensor, the component size is the primary

consideration because space is at a premium inside a chamber.

In this reference design, the default sensor has temperature

and humidity accuracy ratings of ±0.2°C and ±2% relative

humidity, respectively.

Using a Smoke Chamber with the ADPD188BI

Most smoke detector solutions available in the market use

smoke chambers to help reject ambient light, reduce internal

light pollution, and minimize the risk of insects or spiders

interfering with the readings. For the ADPD188BI, using a

smoke chamber causes a constant background signal to appear

in the readings due to light scattering from the chamber surface.

It is critical that the level of the background signal be kept

similar or below the alarm threshold to avoid significant

errors in the readings.

Figure 6. Background Signal Appearing on ADPD188BI Readings

The response of the ADPD188BI to the chamber can also be

expressed in a power transfer ratio (PTR), adding to the

smoke response and must be considered when interpreting

the PTR data of the device.

PTRTOTAL = PTRCHAMBER + PTRSMOKE

(17)

where:

PTRTOTAL is the total power transfer ratio in nW/mW.

PTRCHAMBER is the ratio of the received optical power from the

inside of the chamber surface to the transmitted optical

power, in nW/mW.

PTRSMOKE is the ratio of the received optical power due to

smoke particles scattering the transmitted optical power, in

nW/mW.

Figure 7. Backscattering of Light by the Chamber Walls and Smoke

The ADPD188BI uses the Analog Devices, Inc., proprietary

smoke chamber that is specifically designed to meet device

and industry requirements. The internal geometry of this

smoke chamber allows for the highest signal-to-noise ratio

(SNR) readings and, therefore, optimal PTR values for the

ADPD188BI.

The mechanical design was qualified using stress tests

specified by industry standards, such as JESD22-A101,

JESD22-A103, JESD22-A104, UL 217, UL 268, EN 54, and

AEC-Q100. The chamber is also significantly smaller than

most existing solutions due to the size of the ADPD188BI

relative to discrete smoke detectors. The design only

measures 36 mm across the ends of its two large flanges and

leaves an internal area of 109.36 mm2 underneath it.

Conventional smoke chamber designs are incompatible with

the ADPD188BI because the integrated optoelectronics of the

ADPD188BI use a backscattering system as opposed to the

forward scattering system used by traditional smoke detectors.

Figure 8. ADPD188BI with Smoke Chamber

Anticondensation Heating

For some regions of the world, condensation can be a nuisance

that must be dealt with to maintain the effectiveness of the

smoke detection unit. To address this condensation, CN-0537

provides optional heater resistors and a humidity sensor to

enable custom algorithm development. If dew and condensation

may form on the inside surface of the chamber, it can cause

light scattering that appears as smoke to the system. This

light scattering is particularly problematic in the more

humid, tropical regions of the world where condensation

formation is a common occurrence.

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SAMPLES

BLUE CH3

BLUE CH4

IR CH3

IR CH4

PLED

CHAMBER WALLS

PPD CHAMBER

LIGHT RECEIVED

BY PD FROM THE

CHAMBER WALLS

PPD SMOKE

LIGHT RECEIVED

BY PD FROM SMOKE