Design-In Guide
For Integrating SGP4x Gas Sensors
Sensirion’s SGP4x gas sensor are highly accurate and long-term stable metal-oxide indoor air quality sensors. To take fulladvantage of their outstanding performance, a few design-in rules must be followed. This guide describes an easy-toimplement and affordable design-in to achieve a superior performance. In addition to this document, it is recommended toconsult the Handling and Assembly Instructions for SGPxx Gas Sensors to guarantee the Sensirion’s SGP4x gas sensor’s
specified accuracy.

IntroductionCoupling to ambient

Environmental sensors, unlike standard electricalcomponents, need to be exposed to the environment they
are supposed to sense.
Sensirion’s SGP4x gas sensors are designed andoptimized to measure indoor air quality. The Sensirion
SGP4x gas sensors come with a lot of functionalities toensure high performance and minimize work on your side.
To experience and make use of all these functionalities, it
is advised to follow the steps described below precisely andaccurately. It is recommended at this point to read thedocumentation available for the SGP4x to fully understandthe concepts of indoor air quality and the working principles
of Sensirion’s SGP4x gas sensor.

Ideally, the sensor is placed as close as possible to thedevice’s outer shell with a large opening allowing the sensor
to be exposed to the ambient. The larger the opening, thebetter the air exchange between the sensor’s direct
surrounding and the ambient. Design restrictions often limit
the opening and finding a placement where an opening
does not harm the design is not always possible, see
Figure 2 for some opening options.

Figure 1 Outline of the document

Figure 2 Influence of openings.

This document guides the reader through the wholeprocess of integrating the SGP4x gas sensor best. It starts
with the definition of an acceptable sensor position anddescribes the optimal integration into the device. In a thirdstep, a quick sanity check is presented to get an idea on
how well the chosen design performs and finally a few
points about the finalization of the design in are presented.

Devices stand on the table or are wall mounted. Suchsurfaces can be a source of VOCs which are emitted, e.g.,
from paints, cleaning agents, or polishes. It is therefore
important that the sensor is faced towards the ambientenvironment and not directly to such a surface to avoid
potential false readings. If, for design or practical reasons,a facing towards a surface cannot be avoided, it is advisedthat the distance between surface and the device’s sensor
opening is at least in the range of 1 cm (~0.5 in.).

1. Placement of Sensor
In order to benefit most from the outstanding sensor
performance, it is important to integrate the sensor at aposition that allows maximum exposure to the ambient. Thisis important because the sensor is supposed to measure
the surrounding air at minimum disturbance due to thedevice.
Figure 3 Sensor orientation in a device.

Decoupling from internal heat sources
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All Sensirion SGP4x gas sensors are temperature
controlled and do not show a temperature dependency. In
a device however, the sensor is surrounded by material that
often outgases which leads to a background signal. Thisoutgassing of the material increases with temperature. A
higher background signal affects the sensor’s reading byleading to a constant signal and masking small
environmental events. Therefore, the system of theintegrated Sensirion SGP4x gas sensor with thesurrounding, outgassing material is temperaturedependent.
To avoid such a temperature dependence, it is important toplace the sensor in the device’s coldest part. Typically, thelargest heat sources in an electronic device are the CPU,the display, the Wifi module, and batteries.

One of the most important features of a mechanical designin is the sealing. Electronic components, batteries, PCBs,and plastic parts of typical devices emit VOCs. They
accumulate inside the device to high concentrations.
Without any sealing the VOCs emitted by the device canreach the sensor and mask environmental VOC changes. It
is therefore recommended to use a silicone or rubber
sealing compressed between the housing and the PCB or
Flex-PCB. If for any reason no sealing material can beintegrated, it might be sufficient to minimize the gap
between housing and PCB. However, it is required to plan
for a wide and short opening to optimize the coupling to theenvironment.
Surrounding material
The right choice of materials in the vicinity of the sensor isvery important. A material that outgases next to the SGP4x
gas sensor will lead to an unwanted background signal.
Recommended
Not recommended Hardplastics PA(e.g., ABS, PC)
 POM
 PTFE/Teflon
 PMMA Peek Metals

Figure 4 Placing the sensor in the coldest part of the device.

To profit from convection produced by internal heatsources, it is further recommended to place the sensor inthe lower part of the device.

2. Integration of the Sensor

Having an appropriate opening and position for the sensors
is only the first step. In a second step, the sensor design-in
must be optimized to take full advantage of the opening.
The main goal of the mechanical design-in is to ensure afast and accurate sensor response to environmental
changes at minimum cross talk due to device-internal
outgassing. Key features of an optimal mechanical designin are a small distance between opening and sensor as wellas good sealing towards the device interior. Design-insuggestions will be discussed in the following paragraphs.
Distance to opening
For good coupling to the environment and a fast responsetime it is important to keep the distance between opening
and sensor as small as possible. Facing the sensor’s top
surface against the opening further minimizes the distance
between sensing element and environment.

Figure 5 Mechanical design-in recommendations. Key features
are a small distance between opening and sensor as well as agood sealing towards the device interior.

Sealing against device interior
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3. Design-In Sanity Check
After having built a first prototype, a quick sanity check will
give a hint on the design’s performance. Besides theprototype, one needs a reference and a VOC source. We
propose to use a Sensirion evaluation kit equipped with an
SGP4x gas sensor as reference and an ink pen as VOC
source. Place your device and the reference 10 cm / 4 in.
apart and expose the source in the middle.

A large meshed metal grid in front of the sensor’s opening
helps to protect the SGP4x gas sensor against ESD. A TVS
diode on the PCB is another possibility.
Capacitor and pull-up resistors
The VDD pin must be decoupled with an RC element(recommended: 4.7  and 1 F) which must be placed as
close a possible to the SGP4x between these two lines. The
required decoupling for VDDH depends on the power
supply network connected to the sensor, a capacitor of 1
F is recommended. VDD and VDDH must be connected
to one single supply VDD.

Figure 6 Quick functionality test of the design-in.

Use the reference to check if there is a signal. In case there
is not, change the VOC source. In case there is a signal,
check the response of your device. Once a clear increasein signal is visible, remove the source and the check thedevice’s recovery. The table below states what number to
expect from a good design-in.
Reaction times
Design-in
Fast rise (<30 s) and recovery(<1min)Slow rise (>30 s) and recovery(>1 min)Little effect on gas measurement indeviceFigure 8 Electronic integration of the SGP4x sensor.

Furthermore, two pull-up resistors must be introduced topull the signals SDA and SCL high. We propose a 10 kΩ
resistor each. However, the bus capacity requirements
must be taken into account as well.
No dust protection needed
Each SGP4x gas sensor comes with an attached dust
protection membrane, thus no additional membrane isneeded.

4. Finalization of Design-In
Once an adequate design is found, a few additional steps
are needed to finalize the SGP4x gas sensor integration.
ESD protection

Figure 9 Additional protection membranes.

Figure 7 ESD protection.
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If for other reasons an additional protection membrane isadded to the device, please consider using a membrane
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with a large pore size to ensure a fast gas exchange throughthe membrane.

5. Combining SGP4x and SHTxx
In many cases, the SGP4x gas sensor is used along withan SHTxx humidity and temperature sensor. Whereas theSHTxx does not influence the SGP4x gas sensor in anyway, the dissipated heat of the SGP4x can lead to positive
temperature offset for the SHTxx sensor. This section will
describe how to integrate an SHTxx and a SGP4x in thesame device.
For the SHTxx most design-in rules of the SGP4x sensor
also apply. For a more in depth reading, it is recommended
to read SHTxx and STSxx Design Guide.

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To minimize the heat transfer through solid material(conduction), i.e., through the PCB, we advise to use twoflex PCBs, one for each sensor, to connect to the main
PCB. This will minimize the heat transfer throughconduction. If for design or cost reasons, this solution is notpossible, the two sensors should be placed as far apart fromeach other as possible. Experience showed that a distanceof around 2 cm (~0.8 in.) and milling slits in the PCB aroundthe SHTxx is sufficient.
In addition, heat transfer through convection should also beconsidered. Integrating the two sensors in separate
dedicated volumes is beneficial and recommended. If, forsome reason, this is not practical, it needs to be consideredthat warm air raises. Thus the SHTxx sensors needs to be
integrated in a lower spot than the SGP4x gas sensor.
The two sensors come with different I2C addresses.
Therefore, they can be connected to the same serial bus.

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Revision History
DateJune 2020Version
1.0Page(s)–Changes–Headquarters and Subsidiaries
Sensirion AGLaubisruetistr. 50
CH-8712 Staefa ZH
Switzerland

Sensirion Inc., USA
phone: +1 312 690 5858
info-us@sensirion.com
www.sensirion.com

Sensirion Korea Co. Ltd.
phone: +82 31 337 7700~3
info-kr@sensirion.com
www.sensirion.com/kr

phone: +41 44 306 40 00
fax:+41 44 306 40 30
info@sensirion.com
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Sensirion Japan Co. Ltd.
phone: +81 3 3444 4940
info-jp@sensirion.com
www.sensirion.com/jp

Sensirion China Co. Ltd.
phone: +86 755 8252 1501
info-cn@sensirion.com
www.sensirion.com/cn

Sensirion Taiwan Co. Ltd
phone: +886 3 5506701
info@sensirion.com
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To find your local representative, please visit
www.sensirion.com/distributors

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