Technical Description SEK-SVM4x
Evaluation Kit for SGP40 and SGP41 – Indoor Air Quality Sensor for VOC and NOx
Measurements

 Sensor module equipped with SGP41 and SHT40
 On-board microcontroller for signal processing
 I2C and UART interface with digital output signals
 USB-UART cable, jumper wire cable set Power supply 3.0–5.5 V
 Plug-and-play for fast evaluation and prototyping

www.sensirion.com

version 1.2 – January 2022

1/12

Table of Contents
1 General Description....................................................................................................................................................3
2 Specifications .............................................................................................................................................................4
3 Sensor Output Signal Description ..............................................................................................................................7

www.sensirion.com

version 1.2 – January 2022

2/12

1 General Description
Specifications can be found in the corresponding datasheets of both components SGP41 and SHT40. The SVM41 as part of theevaluation kit SEK-SVM4x serves as reference design for integrating SGP41, SHT40, and Sensirion’s Gas Index Algorithm toobtain the VOC and NOx Index signals as processed by the on-board microcontroller. It supports all features of both components
and automatically takes care of humidity compensation of the SGP41 raw signal as well as of temperature offset compensationof the SHT40 sensor. Figure 1 visualizes how the signal processing is performed on the SVM41.

Figure 1 Schematic of the signal processing and interface on SVM41.

www.sensirion.com

version 1.2 – January 2022

3/12

2 Specifications
2.1 Electrical Specifications
Table 1 lists electrical specifications of the SVM41. The SVM41 supports both I2C “standard-mode” 1 (see NXP I2C-bus
specification and user manual) and UART.2

Parameter

ValuesSymbol Min.

Typ.

Supply voltage3

VDD

3.0

Idle current

IDD–

Average supply current while NOx
pixel conditioning is active

Average supply current during
measurement
Communication

–
–

Max.

Unit

Comments

3.3 or 5.0 5.5

V

Minimal voltage must be guaranteed also forthe maximum supply current specified in thistable.

2.0

3.0

mA

at 3.3 V supply voltage
The idle mode is activated after power-up as
well as after calling either the
svm41_stop_measurement or the
svm41_device_reset command.

6.5

7.5

mA

at 3.3 V supply voltage
The NOx pixel conditioning is activated for 10 safter calling the svm41_start_measurment
command.

5.0

7.5

mA

at 3.3 V supply voltage

Digital 2-wire interface, I2C or UART

Table 1 Electrical specifications.

2.2 Timing Specifications
Timings in Table 2 refer to the power up and reset of the device and do not reflect the usefulness of the readings.
Parameter

SymbolCondition

Min.

Typ.

Max.

Unit

Power-up timetPU

After hard reset, VDD ≥ 3.0 V

–

25

100

ms

I2C clock frequency

fI2C

–

–

–

100

kHz

UART baud rate

fUART

–

–

115’200–

bits s−1

Recommended readout interval

–

–

0.5

1

60

s

Table 2 Timing specifications.

1 See I2C Interface Description SVM41.

2 Universal Asynchronous Receiver Transmitter. See UART Interface Description SVM41.
3 Any supply voltage as defined by the min./max. range may be used for continuous operation of the sensor. The typical value specifies the supply voltage at which outgoing

quality control is performed.

www.sensirion.com

version 1.2 – January 2022

4/12

2.3 Recommended Operating and Storage Conditions
Please, refer to the SGP41 Datasheet. To ensure an optimal performance of the SVM41 also consider the Handling and Assembly
Instructions for both SGPxx Gas Sensors and SHTxx Humidity Sensors as well as the corresponding Design-in Guides.

2.4 Absolute Minimum and Maximum Ratings
Stress levels beyond those listed in Table 3 may cause permanent damage to the device. Exposure to minimum/maximum rating
conditions for extended periods may affect sensor performance and reliability of the device.
Parameter

RatingSupply voltage VDD−0.3 to +5.5 V

Short-term storage temperature range4−40 to +70 °C

Operating temperature range−20 to +55 °C

Short-term storage humidity range4

0 to 80 % (non-condensing)Operating humidity range

0 to 90 % (non-condensing)Table 3 Absolute minimum and maximum ratings.

2.5 Hardware Interface Specifications
Pin

Name

Comments

1

VDD

Supply voltage

2

GND

Ground3

RX

UART: receiving pin for communication

SDA

I2C: serial data, bidirectional

TX

UART: transmitting pin for communication

SCL

I2C: Serial clock, bidirectional

SEL

Interface select:• Leave floating or pull to VDD to select UART
• Pull to GND to select I2C

NC

Do not connect

45
6

Table 4 Pin assignment of the Molex connector. Pin colors match with assignment of the jumper wire cables, which is delivered together with
the SEK-SVM4x.

The SVM41 sensor module is equipped with a serial communication interface. The interface connector Molex 5055670671 is
located at the side of the sensor adjacent. The UART-USB cable and the jumper wire cable delivered in the SEK-SVM4x are
equipped with the corresponding female plug, i.e., Molex 5055650601. In Table 4 a description of the pin layout is given. The
SVM41 offers both a UART and an I2C interface. I2C was originally designed to connect two chips on a PCB. When the sensor isconnected to the main PCB via a cable, particular attention must be paid to electromagnetic interference and crosstalk. Use as
short as possible (≤10 cm) and/or well shielded connection cables such as the jumper wire cable part of the SEK-SVM4x.

4 Short-term storage refers to temporary conditions during, e.g., transport.

www.sensirion.com

version 1.2 – January 2022

5/12

Alternatively, it is recommended to use the UART interface instead, which is more robust against electromagnetic interference,
especially with long connection cables.
A typical I2C application circuit is shown in Figure 2. Both SCL and SDA lines are open drain I/Os. They must be connected toexternal pull-up resistors (e.g., Rp = 10 kΩ). Important notice: to correctly select I2C as interface, the interface select pin (SEL)
must be pulled to GND before or at the same time the sensor is powered up.

Figure 2 Typical I2C application circuit to connect the SVM41 sensor module.

A typical UART application circuit is shown in Figure 3.

Figure 3 Typical UART application circuit to connect the SVM41 sensor module.

www.sensirion.com

version 1.2 – January 2022

6/12

3 Sensor Output Signal Description
3.1 Start-up Behavior
The sensor starts powering-up after reaching the power-up threshold voltage of 3.0 V and enters idle mode after max. 100 ms. In
this state, the SVM41 is ready to receive commands by the master.

3.2 Performing a Measurement
To perform a measurement, the measure mode of SVM41 has to be activated from idle mode by calling thesvm41_start_measurement command. The measure mode remains active until either sending the svm41_stop_measurement,
the svm41_device_reset command or until a hard reset is executed. During measure mode, the SVM41 performs a measurement
every second irrespective of the sampling interval applied by the master to get the resulting data. Once in measure mode andafter waiting for the specified time, either svm41_get_signals (Figure 4) or svm41_get_raw_signals (Figure 5) command can becalled to retrieve sensor data. The master will always send back the latest result of the measurement. During the first 45 s after
sending the svm41_start_measurement command, both the VOC and NOx Index will be 0. Thereafter, both indices will show
values ranging from 1 to 500.
The svm41_get_signals command provides VOC and NOx Index as well as T-offset compensated relative humidity andtemperature (Figure 4) while the svm41_get_raw_signals outputs SGP41 raw signals SRAW_VOC and SRAW_NOX as well as
uncompensated relative humidity and temperature (Figure 5). When calling the svm41_start_measurement command, aconditioning of the NOx pixel is performed for 10 s. During this period, the raw signal SRAW_NOX is 0. Thereafter, it will show
values >0.

Figure 4 Process flow for performing a standard measurement with the SVM41 to receive relative humidity, temperature,
VOC Index and NOx Index.

Figure 5 Process flow for performing a measurement including raw signals with the SVM41 to receive uncompensatedrelative humidity, uncompensated temperature, SGP41 raw VOC signal (SRAW_VOC) and SGP41 raw NOx signal(SRAW_NOX).

www.sensirion.com

version 1.2 – January 2022

7/12

3.3 Using the Temperature Offset for Compensating RHT Measurements
The SVM41 provides the possibility to compensate relative humidity and temperature signals of the SHT40 by setting atemperature offset which can be caused by self-heating of the PCB and/or other environmental influences (such as design-ins).
Current temperature offset can be read out by sending the svm41_get_temperature_offset command at any time during idle ormeasure mode (Figure 6 top). To set the offset value, use the svm41_set_temperature_offset command. This command can only
be sent while the SVM41 is in idle mode (Figure 6 bottom). After waiting for the specified time, the master may send thesvm41_start_measurement command to initialize the measure mode. All RHT signals subsequently received via
svm41_get_signals are automatically compensated. The temperature offset is not stored permanently on the SVM41. Hence,
after startup and reset, the parameter must be set again if still needed. To store the parameter permanently thesvm41_store_input_parameters command must be used (see section 3.5 Storing Input Parameters to Non-Volatile Memory).

Figure 6 Implementation of the temperature offset correction for RHT signals on the SVM41.

3.4 Using the Tunability Feature of the Gas Index Algorithm

Figure 7 Implementation of the tunability feature for the Gas Index Algorithm on SVM41. The parameters are read out and/or set via
separate commands for each algorithm, i.e., VOC and NOx Algorithm.

The SVM41 interface provides the possibility to tune the VOC and the NOx Algorithm individually by a set of six different
parameters. Current parameters (as listed in Table 5) can be read out during both idle and measure mode by sending thesvm41_get_voc_parameters and/or svm41_get_nox_parameters command (Figure 7 top). After the specified waiting time, theSVM41 replies with the payload which represents the currently set six available tuning parameters (Table 5). The same
parameters can be set when the master calls the svm41_set_voc_parameters and/or svm41_set_nox_parameters command(Figure 7 bottom). Note: learning time gain hours and initial standard deviation have no impact on the tuning of the NOx Algorithm;
however, they must be specified (see Table 5). All set parameters are not stored permanently on the SVM41. Hence, after startup
and reset parameters must be set again if still needed. To store the parameters permanently the svm41_store_input_parameters
command must be used (see section 3.5 Storing Input Parameters to Non-Volatile Memory).

www.sensirion.com

version 1.2 – January 2022

8/12

Parameter

Name in driver

Meaning

Default
VOC

Default
NOx

Min.

Max.

Unit

Average
Index,
̅Index𝒙

voc_index_offset,
nox_index_offset

Gas Index value to which theaverage conditions shall bemapped.

100

1

1

250

–

Learning
time offset
hours,
𝒕offsetlearning_time_offset_hours

Duration as rolling window used by
the Gas Index Algorithm to estimate
its offset from history. Data longer
than approx. twice the learning time
will be forgotten.

12

12

1

1’000

h

Learning
time gain
hours,𝒕gain

learning_time_gain_hours

Duration as rolling window used by
the Gas Index Algorithm to estimate
its gain from history. Data longer
than approx. twice the learning time
will be forgotten.

12

12 5

1

1’000

h

Maximum
gatingduration,𝒕gatinggating_max_duration_minutes

During this period, the estimator of
the Gas Index Algorithm states isfrozen when the Gas Index is veryhigh. If the Gas Index remains high,the Gas Index Algorithm will start toadapt its states after this period tothis new situation. 0 disables thisfeature.

180

720

0

3000

min

Initialstandard
deviation,
𝝈initial

std_initial

Initial standard deviation is usedduring start-up of the sensor and
Gas Index Algorithm. During thisperiod, a lower value boosts Gas
Index events while a higher value
decreases Gas Index events which
will have an impact on device-todevice variation.

50

50 5

10

5000

–

Gain
factor,𝒇gain

gain_factor

Factor to either amplify or attenuate
the entire Gas Index signal.

230

230

1

1’000

–

Table 5 List of tunable parameters of the Gas Index Algorithm.

3.5 Storing Input Parameters to Non-Volatile Memory
The temperature offset used for compensating the RHT signals via the svm41_set_temperature_offset command as well as theparameters used for tuning the Gas Index Algorithm via the svm41_set_voc_parameters and svm41_set_nox_parameters
commands can be permanently stored to the non-volatile memory of SVM41 by calling the svm41_store_input_parameters
command and a subsequent waiting for max. 500 ms (Figure 8). For this, at least one of the set commands must be called before.
The svm41_store_input_parameters command can be called at any time and during both idle and measure mode. After any hardor soft reset the corresponding algorithms will use the stored parameters. To set the storage back to factory settings, the mastermust overwrite all parameters applying the same set of commands while using the default values (see interface descriptions).

5 Parameter just for consistency purposes. Always set this parameter to the default value.

www.sensirion.com

version 1.2 – January 2022

9/12

Figure 8 How to store the temperature offset used for compensating RHT signals and parameters of the Gas Index Algorithm to the nonvolatile memory of SVM41.

3.6 Using the Memory Feature of the VOC Algorithm

Figure 9 Implementation of the memory feature for the VOC Algorithm on SVM41.

The SVM41 allows the master to store the current states of the VOC Algorithm to resume operation with the same states after aninterruption (e.g., after a blackout). This will skip the initial learning phase of the VOC Algorithm (i.e., the first 90 min after sending
the svm41_start_measurement command while staying in measure mode). To use this feature, the master needs to send thesvm41_get_voc_states command not until the measure mode has been run for at least 3 h (Figure 9). After the specified waiting
time, the SVM41 replies with the payload, which contains the states of the VOC Algorithm. These values must be stored by themaster on an external non-volatile memory together with an absolute timestamp. Once the SVM41 is powered-up and in idle
mode again, the master may send the svm41_set_voc_states command together with the stored states back to the slave. Thisshould be executed only when the stored parameters are not older than 10 minutes. After the specified waiting time, the mastermay call the svm41_start_measurement command to enter the measure mode again. The VOC Algorithm will now use the states
sent by the slave to provide the VOC Index.
Note: The states sent via the svm41_set_voc_states command are only considered for the very beginning after restarting themeasure mode. After that, the estimator will continuously adapt the states on the rolling window as given by the learning times(see section 3.4 Using the Tunability Feature of the Gas Index Algorithm). This feature is only available for the VOC Algorithm.

3.7 Resetting the Device
The SVM41 can be reset by either hard reset or by sending the svm41_reset_device command. In the latter case, the slave will
first reply and after max. 100 ms the SVM41 restarts and enters the idle mode. All previously stored parameters (sent by thesvm41_set_temperature_offset, svm41_set_voc_parameters, svm41_set_nox_parameters, and svm41_set_voc_statescommands) will be erased upon resetting the device. The temperature offset and the Gas Index Algorithm parameters can be
stored to the non-volatile memory of SVM41 by calling the svm41_store_input_parameters command.

www.sensirion.com

version 1.2 – January 2022

10/12

Revision History
Date

VersionPage(s)Changes

October 2021

1.0

All

Initial release

December 2021

1.1

All

Editorial amendments

4

Average supply current during NOx pixel conditioning added to Table 1

9Default value for maximum gating duration for NOx Algorithm in Table 5 changed

9

Added column “Name in driver” to Table 5

January 2022

www.sensirion.com

1.2

version 1.2 – January 2022

11/12

Copyright © 2022 by SENSIRION
CMOSens® is a trademark of Sensirion
All rights reserved

Headquarters and Subsidiaries
Sensirion AG
Laubisruetistr. 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
www.sensirion.com

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
www.sensirion.com

To find your local representative, please visit www.sensirion.com/distributors

www.sensirion.com

version 1.2 – January 2022

12/12