Data Sheet AHT21
16Pages

Humidity and Temperature Sensor • Full calibration • Digital output, I2C interface • Excellent long-term stability • SMD package suitable for reflow soldering • Quick response and strong anti-jamming capability Product Summary As a new generation of temperature and humidity sensors, AHT21 sets a new standard in terms of size and performance: a two-row flat pin less SMD package suitable for reflow soldering, bottom face 3x3mm, height 0.8mm.The sensor outputs calibrated digital signals in standard IIC format. AHT21 is equipped with a newly designed ASIC dedicated chip, an improved MEMS semiconductor capacitive humidity sensor element and a standard on-chip temperature sensor element. Its performance has been greatly improved or even exceeded the reliability level of the previous generation of sensors. A generation of temperature and humidity sensors have been improved to make their performance more stable in harsh environments. Each sensor is calibrated and tested, and the product lot number is printed on the surface of the product. Due to improvements and miniaturization of the sensor, it is more cost-effective, and ultimately all devices will benefit from cutting-edge energy-saving operation modes. 1 Product Description The AHT21 can be widely used in consumer electronics, medical, automotive, industrial, meteorological and other fields, such as: HVAC, dehumidifiers and refrigerators and other home appliances, testing and testing equipment and other related temperature and humidity detection and control products. Figure 1. AHT21 Sensor Package Diagram (Unit: mm Tolerance: ±0.1 mm).

2 Sensor Specifications Relative Humidity Parameter resolution condition Typical condition Typical Table 1. Humidity characteristics table. Temperature Parameter Prolonged Drift Table 2. Temperature Characteristic table Figure 2. Typical and maximum error in relative humidity at 25° C Figure 3. Typical and maximum temperature errors

AHT21 Data Sheet Electrical Specifications Package Information Table 3. Electrical characteristics. 3 Expansion Performance 3.1 Operating Conditions The sensor has stable performance within the recommended working range, as shown in Figure 4. Long-term exposure to conditions outside the normal range, especially when the humidity is> 80%, may cause temporary signal drift (drift + 3% RH after 60 hours). After returning to normal working conditions, the sensor will slowly selfrecover to the calibration state. Refer to "Recovery Processing" in section 4.3 to speed up the recovery process....

AHT21 Data Sheet 3.2 RH Accuracy at Different Temperatures Figure 2 defines the RH accuracy at 25°C, and Figure 5 shows the typical humidity error for other temperature ranges. Figure 5. The typical error of humidity in the range of 0~80°C, unit: (%RH) Please note: The above error is the typical error (excluding hysteresis) of the reference instrument test with a high-precision dew point meter. 3.3 Electrical Characteristics The power consumption given in Table 3 is related to temperature and supply voltage VDD. Refer to Figures 6 and 7 for power consumption estimation. Please note that the...

Figure 7 The relationship between typical supply current and supply voltage at 25℃ Curve (sleep mode), please note that the deviation of these data from the displayed value may reach ±50% of the displayed value. At 60℃, the coefficient is about 15 (Compared with table 2) 4 Applications 4.1 Welding Instructions SMD I/O pads are made of copper lead frame plane substrates, except these pads are exposed and are used for mechanical and circuit connections. For use, both I/O pads and bare pads need to be soldered to the PCB. To prevent oxidation and optimize welding, the solder joints at the...

Figure 9. JEDEC Standard welding procedure diagram. Tp<=260°C，tp<30 sec, lead-free welding. TL<220°C，tl<150 sec, The rate of temperature rise and fall during welding shall be < 5°C/sec Sensor can be welded through standard reflow furnace. The sensor fully meets the IPC/JEDEC J-STD-020D welding standard. The contact time should be less than 30 seconds at the highest 260℃ (see Fig. 9) and the ultimate welding temperature that the sensor can withstand is 260℃，so it is recommended to use low temperature 180℃ when reflow soldering. Note: After reflow welding, the sensor should be stored at room...

AHT21 Data Sheet should be taken to minimize the effect of heat transfer as far as possible in the design of the circuit. For example, to maintain good ventilation of the shell, the copper coating of AHT21 and other parts of the printed circuit board should be as smallest as possible, or leave a gap between them. (See Fig. 10) Figure 10 The top view of AHT21 printed circuit board, the design of the milling slit is added in the figure, The heat transfer can be reduced to a minimum. In addition, when the measurement frequency is too high, the temperature of the sensor itself will rise and...

Figure 14. The sensor is isolated from the internal heat source to minimize the effect of internal heat on sensor. 5) The sensor power supply can be controlled In order to improve the stability of the system, the following power supply controllable scheme is provided: Figure 15. Typical application circuit. Note: 1. The power supply voltage range of the host MCU to the sensor is 2.2~5.5V. 2. When the sensor is just powered on, MCU gives priority to VDD power supply, and SCL and SDA high levels can be set after 5ms. 3. The VDD of the sensor shall be supplemented with RC filtering circuit, as...

Table 5. Sensor pin distribution (top view) 5.1 Power Pin (VDD, GND) The power supply range of the sensor is 2.2-5.5V. A decoupling capacitor of 10uF must be added between VDD and GND to play a filtering role. VDD is better than SDA and SCL power on or synchronous power on, to avoid the signal line (SCL/SDA) leakage current into the chip, resulting in power on the chip in the non-working state. 5.2 Serial Clock SCL The serial clock is used to synchronize the communication between the microprocessor and sensor. Since the interface contains completely static logic, there is no minimum SCL...