ASAIR®

AHT20 Product manuals
Temperature and Humidity sensor•Full calibration•Digital output, I 2 C interface•Excellent long-term stability•SMD package suitable for reﬂow soldering•Quick response and strong anti-jamming capability

AHT20, as a new generation of temperature andhumidity sensors, has established a new standard
in size and intelligence. It is embedded in a double
row ﬂat no-lead package suitable for reﬂow
soldering, with a bottom of 3 x 3 mm and a heightof 1.0 mm. The sensor outputs calibrated digital
signals in standard I 2 C format.AHT20 is equippedwith a newly designed ASIC chip, an improved
MEMS semiconductor capacitive humidity sensing
element and a standard on-chip temperaturesensing element.

3.0
2.0

As a result, the performance of the new generationof temperature and humidity sensors has greatly
improved or even exceeded that of the previous ones
with more stability in harsh environments.
Each sensor is calibrated and tested, with productbatch number printed on the surface of the product.
Due to the improvement and miniaturization of thesensor, its cost-eﬀective ratio is higher, andﬁnally all equipment will beneﬁt from the cuttingedge energy-saving operation mode.

1.1
0.7
0.8
0.4
1.0

ASAIR®

3.0
0.55

AHT20
XXXXXX

0.4

Buttom view

Top view

Top view

Buttom view

2.8±0.03mm
1.0

Front view

Front view

Figure 1: AHT20 Sensor Package Diagram (Unit: mm Tolerance: ±0.1 mm)

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ASAIR®

AHT20 Product manuals△T℃△RH(%R H )±10±8

Typical Value±2.0
Max Value

Max Value

±6

±1.5

±4

±1.0

±2
±0

±0.5
0

10

20

30

40

50

60

70

80

90

100

±0.0

-40

-20

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40

60

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The power consumption given in Table 1 is related totemperature and supply voltage VDD. Estimated power
consumption, see Figures 6 and 7. Note that the curves
in Figures 6 and 7 are typical natural characteristics
and may have deviations.

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AHT20 Product manuals

9

The I/O pads of SMD are made of copper pin frame
planar substrates, which are exposed to the outside
for mechanical and electrical connections. Whenused, I / O pads and bare pads need to be welded on
PCB. In order to prevent oxidation and optimize
welding, the welding joints at the bottom of thesensor are plated with Ni/Au.On PCB, the lengthof I/O contact surface should be 0.2 mm longer thanthat of the I/O package pad of AHT 20. The inner partshould match the shape of the I/O package pad. The
ratio of pin width to SMD package pad width is1:1.
See ﬁgure 8.
For screen and solder layer design, it is suggested
to use copper foil deﬁnition solder (SMD) with thesolder layer opening larger than the metal solder
plate.
For SMD pads, if the gap between the copper
foil pad and the soldering layer is 60m-75m,
the opening size of the soldering layer shall begreater than the size of the soldering plate(120 m-150 m).
The circular part of the sealing pad shall matchthe corresponding circular solder layer opening
to ensure that there is enough solder layer area(especially at the corner) to prevent solder fromjoining.
Each pad shall have its own soldering layer
opening, forming a soldering layer network
around the adjacent pads.
7

2.0

0.5
1.0

tP

TP
TL
Temperature

2.1 Welding Speciﬁcation

Top view

Due to the low SMD mounting, it is recommended to use
no-cleaning type 3 solders tin and to purify it withnitrogen during reﬂux.

tL

T S (max)

Preheating zone

Critical zoneTime

AHT 20 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 theultimate welding temperature that the sensor canwithstand is 260℃，so it is recommended to use lowtemperature 180℃ when reﬂow soldering.
Note: After reﬂow welding, the sensor should be
stored in the environment of > 75% RH for at least
12 hours to ensure the re-hydration of the polymer.
Otherwise, it will cause sensor reading drift. The
sensor can also be placed in a natural environment(> 40% RH) for more than ﬁve days to re-hydrate.
Hydration time can be reduced by using low
temperature reﬂow welding (e.g.180℃).
Don't wash the circuit boards is allowed after
welding. Therefore, it is suggested that customers
use "wash-free" solder paste. If the sensor isapplied to corrosive gases, condensate water may
be produced (e.g. in high humidity environment),
both pin pads and PCB need to be sealed(e.g. using conformal coating) to avoid poor contact
or short circuit.
2.2 Storage conditions and instructions

0.8

For solder printing, laser cutting stainless steel meshwith electronic polishing trapezoidal wall isrecommended, with recommended thickness of
0. 125 mm. The steel mesh size of the pad should be
0.1 mm longer than PCB pad and placed 0.1 mm awayfrom the packaging center. Steel mesh with bare pads
must cover 70% - 90% of the pad area - that is, thecentral position of the heat dissipation area reaches
1. 4 mm x 2. 3 mm.

The humidity sensitivity level (MSL) is 1, according
to IPC/JEDEC J-STD-020 standard. Therefore, it isrecommended to use it within one year after delivery.
Humidity sensor is not an ordinary electronic
component, and it needs careful protection, whichusers must pay attention to. Long-term exposure
to high concentration of chemical vapor will cause
the sensor reading to drift.

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AHT20 Product manuals
Therefore, it is recommended that the sensor be
stored in the original package including sealed ESD
bag, and meet the following conditions: temperature
range 10~50℃（0~85℃ in a limited time), humidity
20~60% RH(no ESD packaged sensor). For sensors
that they be stored in antistatic bags made of metal
PET/AL/CPE.
During production and transportation, sensors should
avoid exposure to high concentration of chemicalsolvents and prolonged exposure. Avoid exposure tovolatile glue, adhesive tape, stickers or volatile
packaging materials, such as foamed foil, foammaterial, etc. The production area should be wellventilated.

2.5 Product application scenario design
In product design, the sensor has following
characteristics:
1) Sensor is in full contact with the outside air
Housing
PCB
Figure 10: Suitable windows on the enclosure provide goodaccess to environmental measurements and allow for greater
air exchange.

2) The sensor is completely isolated from the
air nside the housing
Housing
PCB

2.3 Recovery processing

Figure 12: The sensor is isolated from the air inside the housing,which minimizes the impact of the air inside the housing on thesensor.

As mentioned above, if the sensor is exposed to
extreme working conditions or chemical vapor, thereading will drift. It can be restored to the calibration
state by processing as follows. Drying: Keep for 10hours at 80~85℃ with the humidity of more than 75%
RH.
Rehydration: Keep for 12 hours at 20~30℃ with thehumidity of more than 75% RH.

3) Small measurement dead zone around the sensor
Housing
PCB
Figure 13: Small measurement dead zone helps the sensor toquickly and comprehensively detect environmental changes.

10104) The sensor is isolated from the heat

2.4 Temperature inﬂuence
The relative humidity of gases depends largely on
temperature. Therefore, when measuring humidity, allsensors measuring the same humidity should work at
the same temperature as possible. When testing, it isnecessary to ensure that the same temperature, andthen compare the humidity readings.
If the sensor and the heating-prone electronic
components are placed on the same printing circuitboard, measures should be taken to minimize theeﬀect of heat transfer as far as possible in the designof the circuit.
For example, to maintain good ventilation of the shell,
the copper coating of AHT20 and other parts of theprinted circuit board should be as smallest as
possible, or leave a gap between them. (See Fig. 10)

Moreover, when the measurement frequency is too
high, the temperature of the sensor itself will rise,
which will aﬀect the measurement accuracy. In orderto make its temperature rise below 0.1 , theactivation time of AHT20 should not exceed 10% of
the measurement time - it is recommended tomeasure data every 2 seconds.

Figure 14: The sensor is isolated from the internal heat source tominimize 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 two
solutions for controlling power supply are provided:

Figure 15-1 Typical application circuit ①, the pull-up voltage and
VDD of SCL and SDA are powered by the MCU.
Note: 1. The host MCU supplies AHT20 with a voltage range of 2.0~ 5.5V.
2. When the AHT20 is just powered on, the MCU gives priority
to the VDD power supply, which can be set after 5ms SCL and SDA
are high.

Figure 15-2 Typical application circuit ②, AHT20 operation iscontrolled by whether GND is grounded.
Note: The user can indirectly control the GND and ground bycontrolling the switch module composed of transistors, so that the
AHT20 is powered oﬀ.

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2.6 Material used for sealing and encapsulation

3.2 Serial clock SCL

Many materials absorb moisture and act as buﬀer, which
will increase response time and hysteresis. Therefore, thematerial around the sensor should be carefully seleaed.
Recommended materials are: Metal materials, LCP, POM(Delrin). PTFE (Teﬂon), PE, PEEK, PP, PB, PPS, PSU,
PVDF, and PVF. Material for sealing and bonding(conservative recommendation): It is recommended to use
method of ﬂling epoxy resin or silicone resin for packaging
electronic components. Gases released from thesematerials may also contaminate AHT20 (see 2.2).
Therefore. the sensor should be ﬁnally assembled andplaced in a well-ventilated place, or dried for 24 hours in
an environment of > 50℃, in order to release thecontaminated gas before packaging.

SCL is used to synchronize the communication
between microprocessor and AHT20. Because theinterface contains complete static logic, there is no
minimum SCL frequency.

2.7 Wiring rules and signal integrity
If the SCL and SDA signal lines are parallel and very
close to each other, it may cause signal crosstalk andcommunication failure. The solution is to place VDD
and / or GND between the two signal lines, separate
the signal lines, and use shielded cables. In addition,
reducing the SCL frequency may also improve theintegrity of signal transmission. A 100nF decoupling
capacitor must be added between the power supply
pins (VDD, GND) for ﬁltering. This capacitor should be
as close as possible to the sensor. See the next chapter.

3.3 Serial data SDA
SDA pins are used for data input and output of sensors.
When sending commands to sensors, SDA is valid at
the rising edge of serial clock (SCL), and SDA mustremain stable when SCL is high level.After thedescending edge of SCL, the SDA value can bechanged. To ensure communication safety, the eﬀective
time of SDA should be extended to TSU and THO
respectively before SCL rising edge and after SCL
falling edge-refer to Fig 17. When the data is read fromthe sensor, SDA is valid (TV) after the SCL decreases
and maintains the descent edge of the next SCL.

Figure 16 Typical application circuit
Note: 1. The pull-up voltage of SCL and SDA must be powered by VDD,
and the power supply voltage range is 2.0 ~ 5.5V;
2. Add 100nF decoupling capacitor between VDD and GND;
3. AHT20 can share I2C bus with other I2C devices.

NC

1

6

NC

VDD

2

5

GN D

SCL

3

4

SD A

Top view

3.1 Power Pins (VDD,GND)
The power supply range of AHT20 is 2.0-5.5V, and therecommended voltage is 3.3V. A decoupling capacitorof 100nF must be added between VDD and GND to playa ﬁltering role. VDD is powered on preferentially orsynchronously than SDA and SCL to avoid the leakage
current from the signal line (SCL / SDA) sinking in,causing the chip to be in a non-working state after
power-on.

To avoid signal collision, MCU must only drive SDA andSCL at low levels. An external pull-up resistor(e.g.10kΩ)is needed to lift the signal to a high level. The pull-up
resistance may have been included in the MCU's I/Ocircuit. Detailed information about sensor input/output
characteristics can be obtained by referring to tables 7
and 8.

4.1 Absolute Maximum Rating
The electric specifcations of AHT20 are defned inTable 2. The absolute maximum ratings given in Table 6
are only stress ratings and to provide more information.
Under such conditions, it is not advisable for the device
to perform functional operation. Exposure to absolute
maximum rating or a long time may aﬀect the reliabilityof the sensor.

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AHT20 Product manuals
ESD electrostatic discharge conforms to JEDECJESD22-A114 standard (human body mode ±4kV) andJEDEC JESD22-A115 (machine mode±200V) If the test
condition exceeds the nominal limit, the sensor needs
additional protection circuit.

4.2 Input/output characteristics
Electric specifcations include power consumption, highand low voltage of input and output, voltage of power
supply. In order to make the sensor communicationsmooth, it is important to ensure that the signal design
is strictly limited to the range given in tables 7, 8 and 17.

AHT20 adopts standard I²C protocol to communicate.
For information on the I²C protocol except thefollowing chapters, please refer to the following
website: www.aosong.com for sample reference.

5.1 Start Sensor
Step 1: Make the sensor power on with selected voltage
of VDD power supply voltage (ranging from 2.0V to 5.5V).
When the sensor is powered on, it takes 20 milliseconds
at most (the SCL is high level) to enter idle state, thatis, tobe ready to receive commands sent by MCU.
t

5.2 Timing sequence of start/stop
Each transport sequence starts with the Start stateand ends with the Stop state, as shown in Figures
18 and 19.
Figure 17

70%

SCL

30%

70%

SDA

30%

18

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AHT20 Product manuals

70%

SCL

5.4 Sensor reading process

30%

70%

SDA

30%

19

1. Wait 40ms after power-on. Before reading the temperatureand humidity values, ﬁrst check whether the calibration
enable bit Bit [3] of the status word is 1 (you can get a byte ofstatus word by sending 0x71). If not 1, need to send 0xbe
command (for initialization), this command parameter has
two bytes, the ﬁrst byte is 0x08, the second byte is 0x00,
and then wait for 10ms.
2. Send the 0xAC command directly (trigger measurement).
The parameter of this command has two bytes, the ﬁrst byte
is 0x33 and the second byte is 0x00.

5.3 Send Command
After the transmission is initiated, the ﬁrst byte of thesubsequent I2C transmission includes the 7-bit I2C
device address 0x38 and a SDA direction bit x (read R:‘1’, write W: ‘0’). After the falling edge of the 8th SCL
clock, the SDA pin (ACK) is pulled low to indicate thatthe sensor data reception is normal. After issuing theinitialization command 0xBE and the measurement
command 0xAC, the MCU must wait until themeasurement is completed. The basic commands are
summarized in Table 9. Table 10 shows the status bits
returned from the slave.

3. Wait for 80ms to wait for the measurement to be completed.
If the read status word Bit [7] is 0, it indicates that themeasurement is completed, and then six bytes can be readin a row; otherwise, continue to wait.
4. After receiving six bytes, the next byte is the CRC checkdata, the user can read it as needed, if the receiving end
needs CRC check, then send it after receiving the sixth byte
ACK response, otherwise NACK is sent out, CRC initial value
is 0XFF, CRC8 check polynomial is:

CRC[7:0]=1+x4 + x5 + x 8
5. Calculate the temperature and humidity values.
Note: The calibration status check in the ﬁrst step
only needs to be checked at power-on. No operationis required during the normal acquisition process.
Trigger measurement data

I²C address + write

Trigger measurement 0xAC

DATA0

DATA0
Read temperature and humidity data

I²C address + read

State

Humidity data

Humidity data

Humidity temperature

Temperature data

Temperature data

CRC data

NACK

[2:0]

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AHT20 Product manuals

5.5 Soft ResetThis command (see Table 11) is used to restart the sensor
system without turning the power oﬀ and on again. After
receiving this command, the sensor system begins to
re-initialize and restore the default setting state, and thetime required for soft reset does not exceed 20 ms.

AHT20 provides SMD packaging (similar to QFN),
which represents a bilateral ﬂat and pin-free package.
The sensor chip is made of a copper lead frame coatedwith Ni/Au. The weight of the sensor is about 19 mg.

All AHT20 sensors have laser labels on their surfaces.
See Figure 20.

ASAIR®
AHT20
XXXXXX

Table 11 Soft Reset– The grey part is controlled by AHT20.

Figure 20: Sensor laser label

6.1 Relative humidity transformation
Relative humidity RH can be calculated according to
the relative humidity signal SRH output from SDA by
the following equation.
(The result is expressed in% RH)

A label is also attached to the tape, as shown in Figure 21,
and other trace information is provided.

®

ASAIR
名称:
Name

温湿度传感器

型号:
Model

AHT20

数量:
Quantity

5000PCS日期:
Date

6.2 Temperature transformation

YYYY-MM-DD

批号:
Batch number

Temperature T can be calculated by substituting thetemperature output signal ST into the following formula.
(The results are expressed as temperature ℃ T)

XXXXXX

2

0

1

9

1

2

1

0

Figure 21: Label on the tape

If the sensor is used in equipment or machinery，please
make sure that it is the same temperature and humiditythat the sensor used for measurement and the sensor
used for reference that have sensed. If the sensor isplaced in the equipment, the reaction time will be
prolonged, so it is necessary to ensure that suﬀcientmeasurement time is reserved in the programming.
The AHT20 sensor is tested according to the enterprisestandard of Aosong temperature and humidity sensor.
The performance of sensors under other test conditions
is not guaranteed and cannot be regarded as a part ofsensor performance. Especially for the speciﬁc
occasions required by users, we do not make any
commitments.
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AHT20 Product manuals

N

13.5±0.5mm

93.0±1mm

W1

A1±1mm

W2

A

I
B

W

F

K

E

Model
AHT20

A1
233/330

Model

Unit
mm

AHT20

P1
E
2

Tolerance
±0.5

A/B

Model

φR1
φR3

+0.1

P2

D

W1
12

W2
16

φR2

N
100

Quantity

Weight5000(AMX)

500/g

K

W

φR2/φR3

+0.1

+0.1

φR1
+0.1

AHT20

3.25 -0.0 1.25 -0.0 12.0±0.3 1.50 -0.0 0.50 -0.0

Model
AHT20

E
P2
I
F
D
P1
8.0±0.1 4.0±0.1 0.3±0.05 5.5±0.1 1.75±0.1 2.0±0.1

Figure 22: Package tape and sensor location diagram

Version information
Date

Version

Page

Change

2019/12

V1.0

1-10

Initial version

2020/04

V1.1

1-11

Add CRC check description, modify application scenario
design and read process description

This manual is subject to change without notice.

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Attention

®

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