SHT40

Part: Sensirion SHT4x

Type: 4th Generation, High-Accuracy, Ultra-Low-Power, 16-bit Relative Humidity and Temperature Sensor Platform

Key Specs:

• Relative humidity accuracy: up to ±1.5 %RH
• Temperature accuracy: up to ±0.1 °C
• Supply voltage: 1.08 V … 3.6 V
• Average current: 0.4 µA (at meas. rate 1 Hz)
• Idle current: 80 nA
• Operating range: 0…100 %RH, -40…125 °C
• RH Resolution: 0.01 %RH
• RH Response time: 6 s (t63%)
• RH Long-term drift: <0.25 %RH/y

Features:

• I2C fast mode plus, CRC checksum
• Fully functional in condensing environment
• Variable power heater (three heating levels)
• NIST traceability
• JEDEC JESD47 qualification
• Mature technology from global market leader
• Several preconfigured I2C addresses

Applications:

• (None explicitly stated)

Package:

• Four-pin dual-flat-no-leads package

• Relative humidity accuracy: up to ±1.5 %RH

• Temperature accuracy: up to ±0.1 °C

• Supply voltage: 1.08 V … 3.6 V

• Average current: 0.4 µA (at meas. rate 1 Hz)

• Idle current: 80 nA

• I2C fast mode plus, CRC checksum

• Operating range: 0…100 %RH, -40…125 °C

• Fully functional in condensing environment

• Variable power heater

• NIST traceability

• JEDEC JESD47 qualification

• Mature technology from global market leader

Figure 11 Pin assignment (transparent top view). Dashed lines are only visible if sensor is viewed from below. The die pad is not directly connected to any pin.

The laser marking consists of two lines, indicated in Figure 11. In the first line a filled circle serves as pin-1 indicator and is followed by "SH4". The last character will indicate the accuracy class of this product (here "x" serves as place holder). In the second line, the first three characters specify the product characteristics according to positions 7,8 and 9 of Table 9. The second three characters serve as internal batch tracking code.

Valid for all electrical specifications: Typical values correspond to VDD = 3.3 V and T = 25 °C. Min. and max. values are valid in the full temperature range -40 °C … 125 °C and at declared VDD levels.

3.1 Electrical Characteristics

Table 3: Electrical specifications.

3.2 Timings

Max. values are measured at -40°C and 1.08 V supply voltage (based on characterization).

Table 4 System timing specifications.

3.3 Absolute Maximum Ratings

Stress levels beyond those listed in Table 5 may cause permanent damage or affect the reliability of the device. These are stress ratings only and functional operation of the device at these conditions is not guaranteed. Ratings are only tested each at a time.

Table 5: Absolute maximum ratings.

4 Sensor Operation

4.1 I2C communication

I2C communication is based on NXP's I2C-bus specification and user manual UM10204, Rev.6, 4 April 2014. Supported I2C modes are standard, fast mode, and fast mode plus. Data is transferred in multiples of 16-bit words and 8-bit check sum (cyclic redundancy check = CRC). All transfers must begin with a start condition (S) and terminate with a stop condition (P). To finish a read transfer, send not acknowledge (NACK) and stop condition (P). Addressing a specific slave device is done by sending its 7-bit I2C address followed by an eighth bit, denoting the communication

^&lt;sup>10 The recommended storage temperature range is 10-50°C. Please consult the document "SHTxx Handling Instructions" for more information.

direction: "zero" indicates transmission to the slave, i.e. "write", a "one" indicates a "read" request. Schematics of the I2C transfer types are sketched in Figure 8.

Figure 8: I2C transfer types: First a write header is sent to the I2C slave, followed by a command, for example "measure RH&T with highest precision". After the measurement is finished the read request directed to this I2C slave will be acknowledged and transmission of data will be started by the slave.

4.2 Data type & length

I2C bus operates with 8-bit data packages. Information from the sensor to the master has a checksum after every second 8-bit data package.

Humidity and temperature data will always be transmitted in the following way: The first value is the temperature signal (2 * 8-bit data + 8-bit CRC), the second is the humidity signal (2 * 8-bit data + 8-bit CRC).

4.3 Checksum Calculation

For read transfers each 16-bit data is followed by a checksum with the following properties

Table 6 Data check sum properties.

The master may abort a read transfer after the 16-bit data, if it does not require a checksum.

4.4 Command Overview

Table 7 Overview of I2C commands.

4.5 Conversion of Signal Output

The digital sensor signals correspond to following humidity and temperature values:

$$RH = \left(-6 + 125 \cdot \frac{S_{RH}}{2^{16} - 1}\right) % RH \tag{1}$$

$$T = \left(-45 + 175 \cdot \frac{S_T}{2^{16} - 1}\right) \circ C \tag{2}$$

$$T = \left(-49 + 315 \cdot \frac{S_T}{2^{16} - 1}\right) \circ F \tag{3}$$

N.B.: The RH conversion formula (1) allows values to be reported which are outside of the range of 0 %RH … 100 %RH. Relative humidity values which are smaller than 0 %RH and larger than 100 %RH are non-physical, however these "uncropped" values might be found beneficial in some cases (e.g. when the distribution of the sensors at the measurement boundaries are of interest). For all users who don't want to engage in evaluation of these non-physical values, cropping of the RH signal to the range of 0 %RH … 100 %RH is advised.

4.6 Serial number

Each and every single sensor has a unique serial number, that is assigned by Sensirion during production. It is stored in the one-time-programmable memory and cannot be manipulated after production. The serial number is accessible via I2C and is transmitted as two 16-bit words, each followed by an 8-bit CRC.

4.7 Reset

A reset of the sensor can be achieved in three ways:

• Soft reset: send the reset command described in Table 7.
• I2C general call: all devices on I2C bus are reset by sending the command 0x06 to the I2C address 0x00.
• Power down (incl. pulling SCL and SDA low)

4.8 Heater Operation

The sensor incorporates an integrated on-chip heater which can be switched on by the set of commands given in Table 7. There are three different heating powers and two different heating times accessible to the user. After reception of a heater-on command, the sensor executes the following procedure:

• 1. The heater is enabled and the timer starts its count-down
• 1. On timer expiration a temperature and humidity measurement with the highest repeatability is started, the heater remains enabled
• 1. After the measurement is finished the heater is turned off
• 1. Temperature and humidity values are now available

The maximum on-time of the heater commands is 1 second, in order to prevent overheating of the sensor by unintended usage of the heater. Thus, there is no dedicated command to turn off the heater. For extended heating periods it is required to send periodic heater-on commands, keeping in mind that the heater is designed for a maximal duty cycle of less than 5%. To obtain a fast increase in temperature the idle time between consecutive heating pulses shall be kept minimal.

Possible Heater Use Cases

There will be dedicated Sensirion application notes elaborating on various use cases of the heater. In general, the applications of the on-chip heater range around:

• 1. Removal of condensed / spray water on the sensor surface. Although condensed water is not a reliability / quality problem to the sensor, it will however make the sensor nonresponsive to RH changes in the air as long as there is liquid water on the surface.
• 1. Creep-free operation in high humid environments. Periodic heating pulses allow for creepfree high-humidity measurements for extended times.

Important notes for operating the heater:

• 1. The heater is designed for a maximum duty cycle of 5%.
• 1. During operation of the heater, sensor specifications are not valid.
• 1. The temperature sensor can additionally be affected by the thermally induced mechanical stress, offsetting the temperature reading from the actual temperature.
• 1. The sensor's temperature (base temperature + temperature increase from heater) must not exceed Tmax = 125 °C in order to have proper electrical functionality of the chip.

If higher heating temperatures are desired, consecutive heating commands have to be sent to the sensor. The heater shall only be operated in ambient temperatures below 65°C else it could drive the sensor outside of its maximal operating temperature.

Stress levels beyond those listed in Table 5 may cause permanent damage or affect the reliability of the device. These are stress ratings only and functional operation of the device at these conditions is not guaranteed. Ratings are only tested each at a time.

Table 5: Absolute maximum ratings.

4 Sensor Operation

4.1 I2C communication

I2C communication is based on NXP's I2C-bus specification and user manual UM10204, Rev.6, 4 April 2014. Supported I2C modes are standard, fast mode, and fast mode plus. Data is transferred in multiples of 16-bit words and 8-bit check sum (cyclic redundancy check = CRC). All transfers must begin with a start condition (S) and terminate with a stop condition (P). To finish a read transfer, send not acknowledge (NACK) and stop condition (P). Addressing a specific slave device is done by sending its 7-bit I2C address followed by an eighth bit, denoting the communication

^&lt;sup>10 The recommended storage temperature range is 10-50°C. Please consult the document "SHTxx Handling Instructions" for more information.

direction: "zero" indicates transmission to the slave, i.e. "write", a "one" indicates a "read" request. Schematics of the I2C transfer types are sketched in Figure 8.

Figure 8: I2C transfer types: First a write header is sent to the I2C slave, followed by a command, for example "measure RH&T with highest precision". After the measurement is finished the read request directed to this I2C slave will be acknowledged and transmission of data will be started by the slave.

4.2 Data type & length

I2C bus operates with 8-bit data packages. Information from the sensor to the master has a checksum after every second 8-bit data package.

Humidity and temperature data will always be transmitted in the following way: The first value is the temperature signal (2 * 8-bit data + 8-bit CRC), the second is the humidity signal (2 * 8-bit data + 8-bit CRC).

4.3 Checksum Calculation

For read transfers each 16-bit data is followed by a checksum with the following properties

Table 6 Data check sum properties.

The master may abort a read transfer after the 16-bit data, if it does not require a checksum.

4.4 Command Overview

Table 7 Overview of I2C commands.

4.5 Conversion of Signal Output

The digital sensor signals correspond to following humidity and temperature values:

$$RH = \left(-6 + 125 \cdot \frac{S_{RH}}{2^{16} - 1}\right) % RH \tag{1}$$

$$T = \left(-45 + 175 \cdot \frac{S_T}{2^{16} - 1}\right) \circ C \tag{2}$$

$$T = \left(-49 + 315 \cdot \frac{S_T}{2^{16} - 1}\right) \circ F \tag{3}$$

N.B.: The RH conversion formula (1) allows values to be reported which are outside of the range of 0 %RH … 100 %RH. Relative humidity values which are smaller than 0 %RH and larger than 100 %RH are non-physical, however these "uncropped" values might be found beneficial in some cases (e.g. when the distribution of the sensors at the measurement boundaries are of interest). For all users who don't want to engage in evaluation of these non-physical values, cropping of the RH signal to the range of 0 %RH … 100 %RH is advised.

4.6 Serial number

Each and every single sensor has a unique serial number, that is assigned by Sensirion during production. It is stored in the one-time-programmable memory and cannot be manipulated after production. The serial number is accessible via I2C and is transmitted as two 16-bit words, each followed by an 8-bit CRC.

4.7 Reset

A reset of the sensor can be achieved in three ways:

• Soft reset: send the reset command described in Table 7.
• I2C general call: all devices on I2C bus are reset by sending the command 0x06 to the I2C address 0x00.
• Power down (incl. pulling SCL and SDA low)

4.8 Heater Operation

The sensor incorporates an integrated on-chip heater which can be switched on by the set of commands given in Table 7. There are three different heating powers and two different heating times accessible to the user. After reception of a heater-on command, the sensor executes the following procedure:

• 1. The heater is enabled and the timer starts its count-down
• 1. On timer expiration a temperature and humidity measurement with the highest repeatability is started, the heater remains enabled
• 1. After the measurement is finished the heater is turned off
• 1. Temperature and humidity values are now available

The maximum on-time of the heater commands is 1 second, in order to prevent overheating of the sensor by unintended usage of the heater. Thus, there is no dedicated command to turn off the heater. For extended heating periods it is required to send periodic heater-on commands, keeping in mind that the heater is designed for a maximal duty cycle of less than 5%. To obtain a fast increase in temperature the idle time between consecutive heating pulses shall be kept minimal.

Possible Heater Use Cases

There will be dedicated Sensirion application notes elaborating on various use cases of the heater. In general, the applications of the on-chip heater range around:

• 1. Removal of condensed / spray water on the sensor surface. Although condensed water is not a reliability / quality problem to the sensor, it will however make the sensor nonresponsive to RH changes in the air as long as there is liquid water on the surface.
• 1. Creep-free operation in high humid environments. Periodic heating pulses allow for creepfree high-humidity measurements for extended times.

Important notes for operating the heater:

• 1. The heater is designed for a maximum duty cycle of 5%.
• 1. During operation of the heater, sensor specifications are not valid.
• 1. The temperature sensor can additionally be affected by the thermally induced mechanical stress, offsetting the temperature reading from the actual temperature.
• 1. The sensor's temperature (base temperature + temperature increase from heater) must not exceed Tmax = 125 °C in order to have proper electrical functionality of the chip.

If higher heating temperatures are desired, consecutive heating commands have to be sent to the sensor. The heater shall only be operated in ambient temperatures below 65°C else it could drive the sensor outside of its maximal operating temperature.

The sensor shows best performance when operated within the recommended normal temperature and humidity range of 5 °C … 60 °C and 20 %RH … 80 %RH, respectively. Long term exposure to conditions outside recommended normal range, especially at high relative humidity, may temporarily offset the RH signal (e.g. +3 %RH after 60 h at > 80 %RH). After returning into the recommended normal temperature and humidity range the sensor will recover to within specifications by itself. Prolonged exposure to extreme conditions may accelerate ageing.

To ensure stable operation of the humidity sensor, the conditions described in the document (Sensirion, 2020) regarding exposure to volatile organic compounds have to be met. Please note as well that this does apply not only to transportation and manufacturing, but also to operation of the SHT4x.

Table 8 Typical values for thermal metrics. In the "heater on" columns a heater power of 200 mW was assumed. Soldering of the die pad is not recommended, therefore the two right hand side columns are bold. Values are based on simulation.