SENSIRION SCD4 Series User manual

www.sensirion.com Version 1.5 –July 20231/27

Product Variants

▪SCD40: Base accuracy, specified measurement

range 400 –2’000 ppm

▪SCD41: High accuracy, specified measurement

range 400 –5’000 ppm, compatible with relevant

IAQ standards, several power modes

SCD4x

Breaking the size barrier in CO2sensing

Products

Details

SCD40-D-R2

Base accuracy, specified range

400 –2’000 ppm

SCD41-D-R2

High accuracy, specified range

400 –5’000 ppm, low power

modes supported

Functional Block Diagram

Product Overview

Product Summary

The SCD4x is Sensirion’s next generation miniature CO2

sensor. This sensor builds on the photoacoustic NDIR

sensing principle and Sensirion’s patented PASens® and

CMOSens® technology to offer high accuracy at an

unmatched price and smallest form factor. SMD assembly

allows cost- and space-effective integration of the sensor

combined with maximal freedom of design. On-chip signal

compensation is realized with the built-in SHT4x humidity

and temperature sensor.

CO2is a key indicator for indoor air quality (IAQ) as high

levels compromise humans’ cognitive performance and

well-being. The SCD4x enables smart ventilation systems

to regulate ventilation in the most energy-efficient and

human-friendly way. Moreover, indoor air quality monitors

and other connected devices based on the SCD4x can

help maintain low CO2concentration for a healthy,

productive environment.

Features

▪Photoacoustic NDIR sensor technology PASens®

▪Smallest form factor: 10.1 x 10.1 x 6.5 mm3

▪Reflow solderable for cost-effective assembly

▪Digital I2C interface

▪Integrated temperature and humidity sensor

 
 
www.sensirion.com Version 1.5 –July 20232/27 
 
Table of Contents 
 
1Sensor Performance .........................................................................................................................................3 
1 CO2Sensing Performance ..........................................................................................................................3 
2 Humidity Sensing Performance ...................................................................................................................3 
3 Temperature Sensing Performance.............................................................................................................3 
2Specifications ....................................................................................................................................................4 
1 Electrical Specifications...............................................................................................................................4 
2 Absolute Maximum Ratings.........................................................................................................................4 
3 Interface Specifications ...............................................................................................................................5 
4 Timing Specifications...................................................................................................................................6 
5 Material Contents ........................................................................................................................................6 
3Digital Interface Description.............................................................................................................................7 
1 Power-Up and Communication Start ...........................................................................................................7 
2 Data Type & Length ....................................................................................................................................7 
3 Command Sequence Types........................................................................................................................7 
4 SCD4x Command Overview........................................................................................................................8 
5 Basic Commands ........................................................................................................................................9 
6 On-Chip Output Signal Compensation ......................................................................................................10 
7 Field Calibration ........................................................................................................................................13 
8 Low Power Periodic Measurement Mode ..................................................................................................14 
9 Advanced Features ...................................................................................................................................15 
10 Single Shot Measurement Mode (SCD41 Only) ........................................................................................17 
11 Checksum Calculation...............................................................................................................................21 
4Mechanical specifications ..............................................................................................................................22 
1 Package Outline ........................................................................................................................................22 
2 Land Pattern..............................................................................................................................................22 
3 Tape & Reel Package................................................................................................................................23 
4 Moisture Sensitivity Level..........................................................................................................................23 
5 Soldering Instructions................................................................................................................................24 
6 Traceability and Identification ....................................................................................................................24 
5Ordering Information.......................................................................................................................................25 
6Revision History ..............................................................................................................................................26 
 
 
 
 
 

www.sensirion.com Version 1.5 –July 20233/27

1Sensor Performance

1.1 CO2Sensing Performance

Default conditions of 25 °C, 50 %RH, ambient pressure 1013 mbar, periodic measurement and 3.3 V supply voltage apply to

values in the table below, unless otherwise stated.

Parameter

Conditions

Value

CO2output range1

0 –40’000 ppm

SCD40 CO2measurement accuracy2

400 ppm –2’000 ppm

±(50 ppm + 5% of reading)

SCD41 CO2measurement accuracy2

400 ppm –1’000 ppm

±(50 ppm + 2.5% of reading)

1’001 ppm – 2’000 ppm

±(50 ppm + 3% of reading)

2’001 ppm – 5’000 ppm

±(40 ppm + 5% of reading)

Repeatability

Typical

±10 ppm

Response time3

τ63%, typical

60 s

Additional accuracy drift after five years

with automatic self-calibration (ASC)

algorithm enabled4

Typical, 400 –2000 ppm

±(5 ppm + 0.5 % of reading)

Table 1: SCD40 and SCD41 CO2sensor specifications

1.2 Humidity Sensing Performance

Parameter

Conditions

Value

Humidity measurement range

0 %RH –100 %RH

Accuracy (typ.)

15 °C –35 °C, 20 %RH –65 %RH

±6 %RH

-10 °C –60 °C, 0 %RH –100 %RH

±9 %RH

Repeatability

Typical

±0.4 %RH

Response time3

τ63%, typical

90 s

Accuracy drift

<0.25 %RH / year

Table 2: SCD4x humidity sensor specifications

1.3 Temperature Sensing Performance

Parameter

Conditions

Value

Temperature measurement range

- 10 °C –60 °C

Accuracy (typ.)

15 °C –35 °C

± 0.8 °C

-10 °C –60 °C

± 1.5 °C

Repeatability

± 0.1 °C

Response time3

τ63%, typical

120 s

Accuracy drift

< 0.03 °C / year

Table 3: SCD4x temperature sensor specifications5

Exposure to CO2concentrations smaller than 400 ppm can affect the accuracy of the sensor if the ASC is on.

Deviation from a high-precision reference with gas mixtures having a ±2% tolerance. Rough handling, shipping and sensor assembly can temporarily impact the accuracy.

Accuracy can be fully restored through the forced recalibration (FRC) or ASC algorithms at least 5 days after sensor assembly (See Section 3.7)

Time for achieving 63% of a respective step function when operating the SCD41 Evaluation Kit in periodic measurement mode. Response time depends on design-in, signal

update rate and environment of the sensor in the final application.

For proper function of the ASC algorithm, the SCD4x must be exposed to air with CO2concentrations of 400 ppm on a weekly basis. Maximum accuracy drift after five years

estimated from stress tests is ±(5 ppm + 2% of reading). Higher drift values may occur if the sensor is not handled according to its handling instructions.

Design-in of the SCD4x in final application, self-heating of the sensor and the environment around the sensor impacts the accuracy of the RH/T sensor. To realize indicated

specifications, the temperature-offset of the SCD4x inside the customer device must be set correctly (see Section 3.6).

www.sensirion.com Version 1.5 –July 20234/27

2Specifications

2.1 Electrical Specifications

Parameter

Symbol

Conditions

Min.

Typical

Max.

Units

Supply voltage DC6

VDD

2.4

3.3 or 5.0

5.5

Unloaded supply voltage ripple peak to peak7

VRPP

Peak supply current8

Ipeak

VDD = 3.3 V

175

205

VDD = 5 V

115

137

Average supply current for periodic measurement

mode, 1 measurement every 5 seconds

IDD

VDD = 3.3 V

VDD = 5 V

Average supply current for low power periodic

measurement mode, 1 measurement every 30

seconds

IDD

VDD = 3.3 V

3.2

3.5

VDD = 5 V

2.8

Average supply current for single shot mode, 1

measurement every 5 minutes (SCD41 only)9

IDD

VDD = 3.3 V

0.45

0.5

VDD = 5 V

0.36

0.4

Input high level voltage

VIH

0.65 x VDD

1 x VDD

Input low level voltage

VIL

0.3 x VDD

Output low level voltage

VOL

3 mA sink current

0.66

Table 4: SCD4x electrical specifications

2.2 Absolute Maximum Ratings

Stress levels beyond those listed in Table 5 may cause permanent damage to the device. Exposure to minimum/maximum

rating conditions for extended periods may affect sensor performance and device reliability.

Parameter

Conditions

Value

Temperature operating conditions

-10 –60 °C

Humidity operating conditions10

Non-condensing

0 –95 %RH

MSL Level

DC supply voltage

-0.3 V –6.0 V

Max voltage on pins SDA, SCL, GND

-0.3 V –VDD + 0.3 V

Input current on pins SDA, SCL, GND

-280 mA –100 mA

Short term storage temperature11

-40 °C –70 °C

Recommended storage temperature

10 °C –50 °C

ESD HBM (pads and metal cap)

2 kV

ESD CDM

500 V

Maintenance Interval

Maintenance free when the ASC

algorithm12 is used.

None

Sensor lifetime13

Typical operating conditions

>10 years

Table 5: SCD4x operation conditions, lifetime and maximum ratings

Supply voltage must be kept constant for stable sensor operation.

Valid only for the supply voltage without the load of the sensor.

Refers to sustained current.

On-demand measurement with freely adjustable interval. See Section 3.10.

Accuracy can be reduced at relative humidity levels lower than 10%.

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

For proper function of the ASC field-calibration algorithm, the SCD4x must be exposed to air with CO2concentrations of 400 ppm on a weekly basis.

Sensor tested over simulated lifetime of >10 years for indoor environment mission profile.

www.sensirion.com Version 1.5 –July 20235/27

2.3 Interface Specifications

The SCD4x comes in an LGA package (Table 6). The package outline is schematically displayed in Section 4.1. The landing

pattern of the SCD4x can be found in Section 4.2.

Name

Comments

VDD

Supply voltage

VDDH

Supply voltage IR source, must

be connected to VDD on

customer PCB

GND

Ground contact

SDA

I2C Serial data, bidirectional

SCL

I2C Serial clock

DNC

Do not connect, pads must be

soldered to a floating pad on

the customer PCB

Table 6: Pin assignment (top view). The notched corner of the protection membrane serves as a polarity mark to indicate pin 1 location.

VDD and VDDH are used to supply the sensor and must always be kept at the same voltage, i.e. both should be connected to

the same power supply. The combined maximum current drawn on VDD and VDDH is indicated in Table 4. VDD and VDDH

must be connected to each other close to the sensor on the customer PCB.

For the sensor operation, a low noise power supply, such as a low-dropout regulator (LDO), should be chosen which can handle

the peak supply current and voltage ripple peak to peak as specified in Table 4. Due to the sensor’s internal regulation, higher

transient currents (on the order of microseconds) may be observed. These transient currents can be neglected in typical

design-in scenarios due to the parasitic R/L/C of the leads as well as the load regulation characteristics of the supply. Additionally,

to avoid interference with the sensor regulation, the non-loaded supply voltage must not vary by more than 30 mV (e.g. ripples

or drops caused by other loads). Operating the sensor with a separate LDO is recommended.

SCL is used to synchronize the I2C communication between the master (microcontroller) and the slave (sensor). The SDA pin

is used to transfer data to and from the sensor. For safe communication, the timing specifications defined in the I2C manual

must be met. Both SCL and SDA lines should be connected to external pull-up resistors (e.g. Rp = 10 kΩ, see Figure 1). To

avoid signal contention, the microcontroller must only drive SDA and SCL low. For dimensioning resistor sizes please take bus

capacity and communication frequency into account (see example in Section 7.1 of NXPs I2C Manual for more details14). It

should be noted that pull-up resistors may be included in the I/O circuits of microcontrollers.

Figure 1: Typical application circuit (representative and not to scale).

NXP I2C-bus specification and user manual UM10204, Rev.6, 4 April 2014

www.sensirion.com Version 1.5 –July 20236/27

2.4 Timing Specifications

Table 7 lists the timings of the ASIC

Parameter

Condition

Min.

Max.

Unit

Power-up time

After hard reset, VDD ≥2.25 V

Soft reset time

After re-initialization (i.e. reinit)

SCL clock frequency

400

kHz

Table 7: System timing specifications.

2.5 Material Contents

The device is fully REACH and RoHS compliant.

Timing specifications based on the NXP I2C-bus specification and user manual UM10204, Rev.6, 4 April 2014

www.sensirion.com Version 1.5 –July 20237/27

3Digital Interface Description

3.1 Power-Up and Communication Start

The sensor starts powering-up after reaching the power-up threshold voltage VDD,min and will take up to the maximum of the

power-up time to enter the idle state. Once the idle state has been reached, it is ready to receive commands from the master.

Each transmission sequence begins with a START condition (S) and ends with a STOP condition (P) as described in the I2C-

bus specification.

3.2 Data Type & Length

Data sent to and received from the sensor consists of a sequence of 16-bit commands and/or 16-bit words (each to be interpreted

as unsigned integer with the most significant byte transmitted first). Each data word is immediately succeeded by an 8-bit CRC.

In write direction it is mandatory to transmit the checksum. In read direction it is up to the master to decide if it wants to process

the checksum (see Section 3.11).

3.3 Command Sequence Types

All SCD4x commands and data are mapped to a 16-bit address space.

SCD4x

Hex. Code

I2C address

0x62

Table 8:I2C device address

The SCD4x features four different I2C command sequence types: “read I2C sequences”, “write I2C sequences”, “send I2C

command”and “send command and fetch result”sequences. Figure 2 illustrates how the I2C communication for the different

sequence types is built-up.

Figure 2: Command sequence types: “write”, “send command”, “read”, and “send command and fetch result”

For the “read”” or “send command and fetch results” sequences, after writing the address and/or data to the sensor and sending

the ACK bit, the sensor needs the execution time (see Table 9) to respond to the I2C read header with an ACK bit. Hence, it is

required to wait the command execution time before issuing the read header. Commands must not be sent while a preceding

command is being processed.

www.sensirion.com Version 1.5 –July 20238/27

3.4 SCD4x Command Overview

An overview of the available SCD4x commands can be found in Table 9. A detailed description for each command can be found

in the following sections.

Table 9:List of SCD4x sensor commands. The final column (‘During meas.’) indicates whether the command can be executed while a

periodic measurement is running.

Domain

Command

Hex.

Code

I2C sequence type

(see Section 3.3)

Execution

time

[ms]

During

meas.

Basic Commands

Section 3.5

start_periodic_measurement

0x21b1

send command

read_measurement

0xec05

read

yes

stop_periodic_measurement

0x3f86

send command

500

yes

On-chip output signal

compensation

Section 3.6

set_temperature_offset

0x241d

write

get_temperature_offset

0x2318

read

set_sensor_altitude

0x2427

write

get_sensor_altitude

0x2322

read

set_ambient_pressure

0xe000

write

yes

get_ambient_pressure

0xe000

read

yes

Field calibration

Section 3.7

perform_forced_recalibration

0x362f

send command and

fetch result

400

set_automatic_self_calibration_enabled

0x2416

write

get_automatic_self_calibration_enabled

0x2313

read

Low power periodic

measurement mode

Section 3.8

start_low_power_periodic_measurement

0x21ac

send command

get_data_ready_status

0xe4b8

read

yes

Advanced features

Section 3.9

persist_settings

0x3615

send command

800

get_serial_number

0x3682

read

perform_self_test

0x3639

read

10000

perform_factory_reset

0x3632

send command

1200

reinit

0x3646

send command

Single shot

measurement mode

(SCD41 only)

Section 3.10

measure_single_shot

0x219d

send command

5000

measure_single_shot_rht_only

0x2196

send command

power_down

0x36e0

send command

wake_up

0x36f6

send command

set_automatic_self_calibration_initial_period

0x2445

write

get_automatic_self_calibration_initial_period

0x2340

read

set_automatic_self_calibration_standard_period

0x244e

write

get_automatic_self_calibration_standard_period

0x234b

read

www.sensirion.com Version 1.5 –July 20239/27

3.5 Basic Commands

This section lists the basic SCD4x commands that are necessary to start a periodic measurement and subsequently read out

the sensor outputs.

The typical communication sequence between the I2C master (e.g., a microcontroller) and the SCD4x sensor is as follows:

1. The sensor is powered up

2. The I2C master sends a start_periodic_measurement command. The signal update interval is 5 seconds.

3. The I2C master periodically reads out data with the read_measurement command.

4. To put the sensor back to idle mode, the I2C master sends a stop_periodic_measurement command.

While a periodic measurement is running, no other commands must be issued with the exception of read_measurement,

get_data_ready_status, stop_periodic_measurement, set_ambient_pressure and get_ambient_pressure.

3.5.1 start_periodic_measurement

Description: start periodic measurement mode. The signal update interval is 5 seconds.

Write

(hexadecimal)

Input parameter: -

Response parameter: -

Max. command

duration [ms]

length [bytes]

signal conversion

length [bytes]

signal conversion

0x21b1

not applicable

Example: start periodic measurement

Write

0x21b1

(hexadecimal)

Command

Table 10: start_periodic_measurement I2C sequence description

3.5.2 read_measurement

Description: read sensor output. The measurement data can only be read out once per signal update interval as the buffer is

emptied upon read-out. If no data is available in the buffer, the sensor returns a NACK. To avoid a NACK response, the

get_data_ready_status can be issued to check data status (see Section 3.8.2 for further details). The I2C master can abort the

read transfer with a NACK followed by a STOP condition after any data byte if the user is not interested in subsequent data.

Write

(hexadecimal)

Input parameter: -

Response parameter: CO2, Temperature,

Relative Humidity

Max.

command

duration [ms]

length [bytes]

signal conversion

length [bytes]

signal conversion

0xec05

CO2 [ppm] = word[0]

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  

  󰇟󰇠

  

Example: read sensor output (500 ppm, 25 °C, 37 %RH)

Write

0xec05

(hexadecimal)

Command

Wait

1 ms

command execution time

Response

0x01f4

0x7b

0x6667

0xa2

0x5eb9

0x3c

(hexadecimal)

CO2= 500 ppm

CRC of 0x01f4

Temp. = 25 °C

CRC of 0x6667

RH = 37%

CRC of 0x5eb9

Table 11: read_measurement I2C sequence description

www.sensirion.com Version 1.5 –July 202310/27

3.5.3 stop_periodic_measurement

Description: stop periodic measurement mode to change the sensor configuration or to save power. Note that the sensor will

only respond to other commands 500 ms after the stop_periodic_measurement command has been issued.

Write

(hexadecimal)

Input parameter: -

Response parameter: -

Max. command

duration [ms]

length [bytes]

signal conversion

length [bytes]

signal conversion

0x3f86

500

Example: stop periodic measurement

Write

0x3f86

(hexadecimal)

Command

Table 12: stop_periodic_measurement I2C sequence description¨

3.6 On-Chip Output Signal Compensation

The SCD4x features on-chip signal compensation to counteract pressure and temperature effects. Feeding the SCD4x with the

pressure or altitude enables highest accuracy of the CO2output signal across a large pressure range. Setting the temperature

offset improves the accuracy of the relative humidity and temperature output signal. Note that the temperature offset does not

impact the accuracy of the CO2output.

To change or read sensor settings, the SCD4x must be in idle mode. A typical sequence between the I2C master and the SCD4x

is described as follows:

1. If the sensor is operated in a periodic measurement mode, the I2C master sends a stop_periodic_measurement command.

2. The I2C master sends one or several commands to get or set the sensor settings.

3. If settings need to be preserved after power-cycle events, the persist_settings command must be sent (see Section 3.9.1)

4. The I2C master sends a start_periodic_measurement command to set the sensor in the operating mode again.

3.6.1 set_temperature_offset

Description: Setting the temperature offset of the SCD4x inside the customer device allows the user to optimize the RH and T

output signal. Note that the temperature offset can depend on several factors such as the SCD4x measurement mode, self-

heating of close components, the ambient temperature and air flow. Thus, the SCD4x temperature offset should be determined

inside the customer device under its typical operation conditions (including the operation mode to be used in the application)

and in thermal equilibrium. By default, the temperature offset is set to 4 °C. To save the setting to the EEPROM, the

persist_settings (see Section 3.9.1) command must be issued. Equation (1) shows how the characteristic temperature offset can

be obtained. Recommended temperature offset values are between 0 °C and 20 °C.

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    (1)

Write

(hexadecimal)

Input parameter: Offset temperature

Response parameter: -

Max. command

duration [ms]

length [bytes]

signal conversion

length [bytes]

signal conversion

0x241d

󰇟󰇠  󰇟󰇠  



Example: set temperature offset to 5.4 °C

Write

0x241d

0x07e6

0x48

(hexadecimal)

Command

Toffset = 5.4 °C

CRC of 0x7e6

Table 13: set_temperature_offset I2C sequence description

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