Rev. 1.0 3/14 Copyright © 2014 by Silicon La boratories Si7021-A10
Si7021-A10
I2C HUMIDITY AND TEMPERATURE SENSOR
Features
Applications
Description
The Si7021 I2C Humidity and Temperature Sensor is a monolithic CMOS IC
integrating humidity and temperature sensor elements, an analog-to-digital
converter, signal processing, calibration data, and an I2C Interface. The patented
use of industry-standard, low-K polymeric dielectrics for sensing humidity enables
the construction of low-power, monolithic CMOS Sensor ICs with low drift and
hysteresis, and excellent long term stability.
The humidity and temperature sensors are factory-calibrated and the calibration
data is stored in the on-chip non-volatile memory. This ensures that the sensors
are fully interchangeable, with no recalibration or software changes required.
The Si7021 is available in a 3x3 mm DFN package and is reflow solderable. It can
be used as a hardware- and software-compatible drop-in upgrade for existing RH/
temperature sensors in 3x3 mm DFN-6 packages, featuring precision sensing
over a wider range and lower power consumption. The optional factory-installed
cover offers a low profile, convenient means of protecting the sensor during
assembly (e.g., reflow soldering) and throughout the life of the product, excluding
liquids (hydrophobic/oleophobic) and particulates.
The Si7021 offers an accurate, low-power, factory-calibrated digital solution ideal
for measuring humidity, dew-point, and temperature, in applications ranging from
HVAC/R and asset tracking to industrial and consumer platforms.
Precision Relative Humidity Sensor
 ± 3% RH (max), 0–80% RH
High Accuracy Temperature Sensor
±0.4 °C (max), –10 to 85 °C
0 to 100% RH operating range
Up to –40 to +125 °C operating
range
Wide operating voltage
(1.9 to 3.6 V)
Low Power Consumption
150 µA active current
60 nA standby current
Factory-calibrated
I2C Interface
Integrated on-chip heater
3x3 mm DFN Package
Excellent long term stability
Optional factory-installed cover
Low-profile
Protection during reflow
Excludes liquids and particulates
HVAC/R
Thermostats/humidistats
Respiratory therapy
White goods
Indoor weather stations
Micro-environments/data centers
Automotive climate control and
defogging
Asset and goods tracking
Mobile phones and tablets
Patent Protected. Patents pending
Ordering Information:
See page 27.
Pin Assignments
Si7021
DNC
SCL
VDD
1
2
6
5GND
SDA
3 4
DNC
Top View
Si7021-A10
2 Rev. 1.0
Functional Block Diagram
ADC
GND
Humidity
Sensor Control Logic
SCL
Si7021
Tem p
Sensor
1.25V
Ref
I2C Interface SDA
Vdd
Calibration
Memory
Si7021-A10
Rev. 1.0 3
TABLE OF CONTENTS
Section Page
1. Electrical Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
2. Typical Application Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
3. Bill of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
4. Functional Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
4.1. Relative Humidity Sensor Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
4.2. Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
4.3. Prolonged Exposure to High Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
4.4. PCB Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
4.5. Protecting the Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
4.6. Bake/Hydrate Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
4.7. Long Term Drift/Aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
5. I2C Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
5.1. Issuing a Measurement Command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
5.2. Reading and Writing User Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
5.3. Electronic Serial Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
5.4. Firmware Revision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
5.5. Heater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6. Control Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
6.1. Register Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
7. Pin Descriptions: Si7021 (Top View) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
8. Ordering Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
9. Package Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
9.1. Package Outline: 3x3 6-pin DFN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
9.2. Package Outline: 3x3 6-pin DFN with Protective Cover . . . . . . . . . . . . . . . . . . . . . . 29
10. PCB Land Pattern and Solder Mask Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30
11. Top Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
11.1. Si7021 Top Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
11.2. Top Marking Explanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
12. Additional Reference Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Document Change List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
Contact Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
Si7021-A10
4 Rev. 1.0
1. Electrical Specifications
Unless otherwise specified, all min/max specifications apply over the recommended operating conditions.
Table 1. Recommended Operating Conditions
Parameter Symbol Test Condition Min Typ Max Unit
Power Supply VDD 1.9 3.6 V
Operating Temperature TAI and Y grade –40 — +125 °C
Operating Temperature TAG grade –40 — +85 °C
Table 2. General Specifications
1.9 < VDD <3.6 V; TA= –40 to 85 °C (G grade) or –40 to 125 °C (I/Y grade); default conversion time unless otherwise noted.
Parameter Symbol Test Condition Min Typ Max Unit
Input Voltage High VIH SCL, SDA pins 0.7xVDD ——V
Input Voltage Low VIL SCL, SDA pins — — 0.3xVDD V
Input Voltage Range VIN SCL, SDA pins with respect to GND 0.0 — VDD V
Input Leakage IIL SCL, SDA pins — — 1 μA
Output Voltage Low VOL SDA pin; IOL = 2.5 mA; VDD = 3.3 V — — 0.6 V
SDA pin; IOL =1.2mA;
VDD =1.9V
——0.4V
Current
Consumption
IDD RH conversion in progress — 150 180 μA
Temperature conversion in progress — 90 120 μA
Standby, –40 to +85 °C2—0.060.62μA
Standby, –40 to +125 °C2—0.063.8μA
Peak IDD during powerup3—3.54.0mA
Peak IDD during I2C operations4—3.54.0mA
After writing to user registers5—20—μA
Heater Current6IHEAT —3.1 —mA
Notes:
1. Initiating a RH measurement will also automatically initiate a temperature measurement. The total conversion time will
be tCONV(RH) + tCONV(T).
2. No conversion or I2C transaction in progress. Typical values measured at 25 °C.
3. Occurs once during powerup. Duration is <5 msec.
4. Occurs during I2C commands for Reset, Read/Write User Registers, Read EID, and Read Firmware Version. Duration is
<100 µs when I2C clock speed is >100 kHz (>200 kHz for 2-byte commands).
5. IDD after a user register write. Initiating any other subsequent I2C transaction on the same bus (such as the user
register read, starting an RH measurement, or traffic directed at other I2C devices) will transition the device to standby
mode.
6. Additional current consumption when HTRE bit enabled. See section “5.5. Heater” for more information
Si7021-A10
Rev. 1.0 5
Conversion Time1tCONV 12-bit RH — 10 12
ms
11-bit RH — 5.8 7
10-bit RH — 3.7 4.5
8-bit RH — 2.6 3.1
14-bit temperature — 7 10.8
13-bit temperature — 4 6.2
12-bit temperature — 2.4 3.8
11-bit temperature — 1.5 2.4
Powerup Time tPU From VDD ≥ 1.9 V to ready for a
conversion, 25 °C
—1825
ms
From VDD ≥ 1.9 V to ready for a
conversion, full temperature range
——80
After issuing a software reset
command
—515
Table 3. I2C Interface Specifications1
1.9 VDD 3.6 V; TA = –40 to +85 °C (G grade) or –40 to +125 °C (I/Y grade) unless otherwise noted.
Parameter Symbol Test Condition Min Typ Max Unit
Hysteresis VHYS High-to-low versus low-to-
high transition
0.05 x VDD ——V
SCLK Frequency2fSCL — — 400 kHz
SCL High Time tSKH 0.6 — — µs
SCL Low Time tSKL 1.3 — — µs
Start Hold Time tSTH 0.6 — — µs
Notes:
1. All values are referenced to VIL and/or VIH.
2. Depending on the conversion command, the Si7021 may hold the master during the conversion (clock stretch). At
above 100 kHz SCL, the Si7021 may also hold the master briefly for user register and device ID transactions. At the
highest I2C speed of 400 kHz the stretching will be <10 µs.
3. Pulses up to and including 50ns will be suppressed.
Table 2. General Specifications (Continued)
1.9 < VDD <3.6 V; TA= –40 to 85 °C (G grade) or –40 to 125 °C (I/Y grade); default conversion time unless otherwise noted.
Parameter Symbol Test Condition Min Typ Max Unit
Notes:
1. Initiating a RH measurement will also automatically initiate a temperature measurement. The total conversion time will
be tCONV(RH) + tCONV(T).
2. No conversion or I2C transaction in progress. Typical values measured at 25 °C.
3. Occurs once during powerup. Duration is <5 msec.
4. Occurs during I2C commands for Reset, Read/Write User Registers, Read EID, and Read Firmware Version. Duration is
<100 µs when I2C clock speed is >100 kHz (>200 kHz for 2-byte commands).
5. IDD after a user register write. Initiating any other subsequent I2C transaction on the same bus (such as the user
register read, starting an RH measurement, or traffic directed at other I2C devices) will transition the device to standby
mode.
6. Additional current consumption when HTRE bit enabled. See section “5.5. Heater” for more information
Si7021-A10
6 Rev. 1.0
Figure 1. I2C Interface Timing Dia gram
Start Setup Time tSTS 0.6 — — µs
Stop Setup Time tSPS 0.6 — — µs
Bus Free Time tBUF Between Stop and Start 1.3 — — µs
SDA Setup Time tDS 100 — — ns
SDA Hold Time tDH 100 — — ns
SDA Valid Time tVD;DAT From SCL low to data valid — — 0.9 µs
SDA Acknowledge Valid Time tVD;ACK From SCL low to data valid — — 0.9 µs
Suppressed Pulse Width3tSPS 50 — — ns
Table 3. I2C Interface Specifications1 (Continued)
1.9 VDD 3.6 V; TA = –40 to +85 °C (G grade) or –40 to +125 °C (I/Y grade) unless otherwise noted.
Parameter Symbol Test Condition Min Typ Max Unit
Notes:
1. All values are referenced to VIL and/or VIH.
2. Depending on the conversion command, the Si7021 may hold the master during the conversion (clock stretch). At
above 100 kHz SCL, the Si7021 may also hold the master briefly for user register and device ID transactions. At the
highest I2C speed of 400 kHz the stretching will be <10 µs.
3. Pulses up to and including 50ns will be suppressed.
SCL
D7
1/fSCL tSKH
SDA
tSKL
tSTH
D6 D5 D0 R/W ACK
tDS tDH
Start Bit Stop Bit
tBUF
tSTS tVD : ACK
tSPS
tSP
Si7021-A10
Rev. 1.0 7
Table 4. Humidity Sensor
1.9 ≤ VDD ≤ 3.6 V; TA = 30 °C; default conversion time unless otherwise noted.
Parameter Symbol Test Condition Min Typ Max Unit
Operating Range1Non-condensing 0 — 100 %RH
Accuracy2, 3 0 – 80% RH — ±2 ±3 %RH
80 – 100% RH See Figure 2
Repeatability/Noise 12-bit resolution — 0.025 —
%RH RMS
11-bit resolution — 0.05 —
10-bit resolution — 0.1 —
8-bit resolution — 0.2 —
Response Time4τ63% 1m/s airflow — 18 — S
Drift vs. Temperature — 0.05 — %RH/°C
Hysteresis — ±1 — %RH
Long Term Stability3—<0.25 — %RH/yr
Notes:
1. Recommended humidity operating range is 20% to 80% RH (non-condensing) over –10 °C to 60 °C. Prolonged
operation beyond these ranges may result in a shift of sensor reading, with slow recovery time.
2. Excludes hysteresis, long term drift, and certain other factors and is applicable to non-condensing environments only.
See section “4.1. Relative Humidity Sensor Accuracy” for more details.
3. Drift due to aging effects at typical room conditions of 30 °C and 30% to 50% RH. May be impacted by dust, vaporized
solvents or other contaminants, e.g., out-gassing tapes, adhesives, packaging materials, etc. See section “4.7. Long
Term Drift/Aging” .
4. Response time to a step change in RH. Time for the RH output to change by 63% of the total RH change.
Si7021-A10
8 Rev. 1.0
Figure 2. RH Accuracy at 30 °C
Si7021-A10
Rev. 1.0 9
Figure 3. Temperature Accuracy*
*Note: Applies only to I and Y grade devices beyond +85 °C.
Table 5. Temperature Sensor
1.9 ≤ VDD ≤3.6 V; TA= –40 to +85 °C (G grade) or –40 to +125 °C (I/Y grade) default conversion time, unless otherwise noted.
Parameter Symbol Test Condition Min Typ Max Unit
Operating Range I and Y Grade –40 — +125 °C
G Grade –40 — +85 °C
Accuracy1–10 °C< tA< 85 °C — ±0.3 ±0.4 °C
–40 < tA< 125°C Figure3
Repeatability/Noise 14-bit resolution — 0.01 —
°C RMS
13-bit resolution — 0.02 —
12-bit resolution — 0.04 —
11-bit resolution — 0.08 —
Response Time2τ63% Unmounted device — 0.7 — s
Si7021-EB board — 5.1 —
Long Term Stability — 0.01 — °C/Yr
Notes:
1. 14b measurement resolution (default).
2. Time to reach 63% of final value in response to a step change in temperature. Actual response time will vary
dependent on system thermal mass and air-flow.
Si7021-A10
10 Rev. 1.0
Table 6. Thermal Characteristics
Parameter Symbol Test Condition DFN-6 Unit
Junction to Air Thermal Resistance JA JEDEC 2-Layer board,
No Airflow
256 °C/W
Junction to Air Thermal Resistance JA JEDEC 2-Layer board,
1 m/s Airflow
224 °C/W
Junction to Air Thermal Resistance JA JEDEC 2-Layer board,
2.5 m/s Airflow
205 °C/W
Junction to Case Thermal Resistance JC JEDEC 2-Layer board 22 °C/W
Junction to Board Thermal Resistance JB JEDEC 2-Layer board 134 °C/W
Table 7. Absolute Maximum Ratings1
Parameter Symbol Test Condition Min Typ Max Unit
Ambient temperature
under bias
–55 — 125 °C
Storage Temperature2–65 — 150 °C
Voltage on I/O pins –0.3 — VDD+0.3V V
Voltage on VDD with
respect to GND
–0.3 4.2 V
ESD Tolerance HBM — — 2 kV
CDM — — 1.25 kV
MM — — 250 V
Notes:
1. Absolute maximum ratings are stress ratings only, operation at or beyond these conditions is not implied and may
shorten the life of the device or alter its performance.
2. Special handling considerations apply; see application note, “AN607: Si70xx Humidity Sensor Designer’s Guide”.
Si7021-A10
Rev. 1.0 11
2. Typical Application Circuits
The primary function of the Si7021 is to measure relative humidity and temperature. Figure 4 demonstrates the
typical application circuit to achieve these functions.
Figure 4. Typical Application Circuit for Relative Humidity and Temperature Measurement
0.1µF
VDD
SCL
SDA
Si7021
SCL
SDA
1.9 to 3.6V
6
1
2
GND
10k
10k
5
Si7021-A10
12 Rev. 1.0
3. Bill of Materials
Table 8. Typical Application Circuit BOM for Relative Humidity and Temperature Measurement
Reference Description Mfr Part Number Manufacturer
R1 Resistor, 10 k, ±5%, 1/16 W, 0603 CR0603-16W-103JT Venkel
R2 Resistor, 10 k, ±5%, 1/16 W, 0603 CR0603-16W-103JT Venkel
C1 Capacitor, 0.1 µF, 16 V, X7R, 0603 C0603X7R160-104M Venkel
U1 IC, Digital Temperature/humidity Sensor Si7021-A10-GM Silicon Labs
Si7021-A10
Rev. 1.0 13
4. Functional Description
Figure 5. Si7021 Block Diagram
The Si7021 is a digital relative humidity and temperature sensor that integrates temperature and humidity sensor
elements, an analog-to-digital converter, signal processing, calibration, polynomial non-linearity correction, and an
I2C interface all in a single chip. The Si7021 is individually factory-calibrated for both temperature and humidity,
with the calibration data stored in on-chip non-volatile memory. This ensures that the sensor is fully
interchangeable, with no recalibration or changes to software required. Patented use of industry-standard CMOS
and low-K dielectrics as a sensor enables the Si7021 to achieve excellent long term stability and immunity to
contaminants with low drift and hysteresis. The Si7021 offers a low power, high accuracy, calibrated and stable
solution ideal for a wide range of temperature, humidity, and dew-point applications including medical and
instrumentation, high reliability automotive and industrial systems, and cost-sensitive consumer electronics.
While the Si7021 is largely a conventional mixed-signal CMOS integrated circuit, relative humidity sensors in
general and those based on capacitive sensing using polymeric dielectrics have unique application and use
requirements that are not common to conventional (non-sensor) ICs. Chief among those are:
The need to protect the sensor during board assembly, i.e., solder reflow, and the need to subsequently
rehydrate the sensor.
The need to protect the senor from damage or contamination during the product life-cycle.
The impact of prolonged exposure to extremes of temperature and/or humidity and their potential effect on
sensor accuracy.
The effects of humidity sensor “memory”.
Each of these items is discussed in more detail in the following sections.
ADC
GND
Humidity
Sensor Control Logic
SCL
Si7021
Temp
Sensor
1.25V
Ref
I2C Interface SDA
Vdd
Calibration
Memory
Si7021-A10
14 Rev. 1.0
4.1. Relative Humidity Sensor Accuracy
To determine the accuracy of a relative humidity sensor, it is placed in a temperature and humidity controlled
chamber. The temperature is set to a convenient fixed value (typically 25–30 °C) and the relative humidity is swept
from 20 to 80% and back to 20% in the following steps: 20% – 40% – 60% – 80% – 80% – 60% – 40% – 20%. At
each set-point, the chamber is allowed to settle for a period of 60 minutes before a reading is taken from the
sensor. Prior to the sweep, the device is allowed to stabilize to 50%RH. The solid trace in Figure 6, “Measuring
Sensor Accuracy Including Hysteresis,” shows the result of a typical sweep.
Figure 6. Measuring Sensor Accuracy Including Hysteresis
The RH accuracy is defined as the dotted line shown in Figure 6, which is the average of the two data points at
each relative humidity set-point. In this case, the sensor shows an accuracy of 0.25%RH. The Si7021 accuracy
specification (Table 4) includes:
Unit-to-unit and lot-to-lot variation
Accuracy of factory calibration
Margin for shifts that can occur during solder reflow
The accuracy specification does not include:
Hysteresis (typically ±1%)
Effects from long term exposure to very humid conditions
Contamination of the sensor by particulates, chemicals, etc.
Other aging related shifts ("Long-term stability")
Variations due to temperature (see Drift vs. Temperature in Table 4). RH readings will typically vary with
temperature by less than 0.05% C.
Si7021-A10
Rev. 1.0 15
4.2. Hysteresis
The moisture absorbent film (polymeric dielectric) of the humidity sensor will carry a memory of its exposure
history, particularly its recent or extreme exposure history. A sensor exposed to relatively low humidity will carry a
negative offset relative to the factory calibration, and a sensor exposed to relatively high humidity will carry a
positive offset relative to the factory calibration. This factor causes a hysteresis effect illustrated by the solid trace
in Figure 6. The hysteresis value is the difference in %RH between the maximum absolute error on the decreasing
humidity ramp and the maximum absolute error on the increasing humidity ramp at a single relative humidity
setpoint and is expressed as a bipolar quantity relative to the average error (dashed trace). In the example of
Figure 6, the measurement uncertainty due to the hysteresis effect is ±1.0%RH.
4.3. Prolonged Exposure to High Humidity
Prolonged exposure to high humidity will result in a gradual upward drift of the RH reading. The shift in sensor
reading resulting from this drift will generally disappear slowly under normal ambient conditions. The amount of
shift is proportional to the magnitude of relative humidity and the length of exposure. In the case of lengthy
exposure to high humidity, some of the resulting shift may persist indefinitely under typical conditions. It is generally
possible to substantially reverse this affect by baking the device (see section “4.6. Bake/Hydrate Procedure” ).
4.4. PCB Assembly
4.4.1. Soldering
Like most ICs, Si7021 devices are shipped from the factory vacuum-packed with an enclosed desiccant to avoid
any RH accuracy drift during storage and to prevent any moisture-related issues during solder reflow. The following
guidelines should be observed during PCB assembly:
Si7021 devices are compatible with standard board assembly processes. Devices should be soldered
using reflow per the recommended card reflow profile. See Section “10. PCB Land Pattern and Solder
Mask Design” for the recommended card reflow profile.
A "no clean" solder process is recommended to minimize the need for water or solvent rinses after
soldering. Cleaning after soldering is possible, but must be done carefully to avoid impacting the
performance of the sensor. See AN607 for more information on cleaning.
It is essential that the exposed polymer sensing film be kept clean and undamaged. This can be
accomplished by careful handling and a clean, well-controlled assembly process. When in doubt or for
extra protection, a heat-resistant, protective cover such as Kapton™ KPPD-1/8 polyimide tape can be
installed during PCB assembly.
Si7021s may be ordered with a factory-fitted, solder-resistant protective cover. This cover provides protection
during PCB assembly or rework but without the time and effort required to install and remove the Kapton tape. It
can be left in place for the lifetime of the product, preventing liquids, dust or other contaminants from coming into
contact with the polymer sensor film. See Section “8. Ordering Guide” for a list of ordering part numbers that
include the cover.
Si7021-A10
16 Rev. 1.0
4.4.2. Rehydration
The measured humidity value will generally shift slightly after solder reflow. A portion of this shift is permanent and
is accounted for in the accuracy specifications in Table 4. After soldering, an Si7021 should be allowed to
equilibrate under controlled RH conditions (room temperature, 45–55%RH) for at least 48 hours to eliminate the
remainder of the shift and return the device to its specified accuracy performance.
4.4.3. Rework
To maintain the specified sensor performance, care must be taken during rework to minimize the exposure of the
device to excessive heat and to avoid damage/contamination or a shift in the sensor reading due to liquids, solder
flux, etc. Manual touch-up using a soldering iron is permissible under the following guidelines:
The exposed polymer sensing film must be kept clean and undamaged. A protective cover is
recommended during any rework operation (Kapton® tape or the factory installed cover).
Flux must not be allowed to contaminate the sensor; liquid flux is not recommended even with a cover in
place. Conventional lead-free solder with rosin core is acceptable for touch-up as long as a cover is in
place during the rework.
If possible, avoid water or solvent rinses after touch-up. Cleaning after soldering is possible, but must be
done carefully to avoid impacting the performance of the sensor. See AN607 for more information on
cleaning.
Minimize the heating of the device. Soldering iron temperatures should not exceed 350 °C and the contact
time per pin should not exceed 5 seconds.
Hot air rework is not recommended. If a device must be replaced, remove the device by hot air and solder
a new part in its place by reflow following the guidelines above.
*Note: All trademarks are the property of their respective owners.
Figure 7. Si7021 with Factory-Installed Protective Cover
Si7021-A10
Rev. 1.0 17
4.5. Protecting the Sensor
Because the sensor operates on the principal of measuring a change in capacitance, any changes to the dielectric
constant of the polymer film will be detected as a change in relative humidity. Therefore, it is important to minimize
the probability of contaminants coming into contact with the sensor. Dust and other particles as well as liquids can
affect the RH reading. It is recommended that a cover is employed in the end system that blocks contaminants but
allows water vapor to pass through. Depending on the needs of the application, this can be as simple as plastic or
metallic gauze for basic protection against particulates or something more sophisticated such as a hydrophobic
membrane providing up to IP67 compliant protection.
The Si7021 may be ordered with a factory-fitted, solder-resistant cover that can be left in place for the lifetime of
the product. It is very low-profile, hydrophobic and oleophobic, and excludes particulates down to 0.35 microns in
size. See section “8. Ordering Guide ” for a list of ordering part numbers that include the cover. A dimensioned
drawing of the IC with the cover is included in section “9. Package Outline ” . Other characteristics of the cover are
listed in Table 9.
4.6. Bake/Hydrate Procedure
After exposure to extremes of temperature and/or humidity for prolonged periods, the polymer sensor film can
become either very dry or very wet, in each case the result is either high or low relative humidity readings. Under
normal operating conditions, the induced error will diminish over time. From a very dry condition, such as after
shipment and soldering, the error will diminish over a few days at typical controlled ambient conditions, e.g., 48
hours of 45 ≤ %RH ≤ 55. However, from a very wet condition, recovery may take significantly longer. To accelerate
recovery from a wet condition, a bake and hydrate cycle can be implemented. This operation consists of the
following steps:
Baking the sensor at 125 °C for ≥ 12 hours
Hydration at 30 °C in 75% RH for ≥ 10 hours
Following this cycle, the sensor will return to normal operation in typical ambient conditions after a few days.
4.7. Long Term Drift/Aging
Over long periods of time, the sensor readings may drift due to aging of the device. Standard accelerated life
testing of the Si7021 has resulted in the specifications for long-term drift shown in Table 4 and Table 5. This
contribution to the overall sensor accuracy accounts only for the long-term aging of the device in an otherwise
benign operating environment and does not include the effects of damage, contamination, or exposure to extreme
environmental conditions.
Table 9. Specifications of Protective Cover
Parameter Value
Material ePTFE
Water Entry Pressure 2.7 bar
Pore Size 0.35 µ
Operating Temperature –40 to 125 °C
Maximum Reflow Temperature 260 °C
Oleophobicity (AATCC 118-1992) 7
IP Rating (per IEC 529) IP67
Si7021-A10
18 Rev. 1.0
5. I2C Interface
The Si7021 communicates with the host controller over a digital I2C interface. The 7-bit base slave address is
0x40.
Master I2C devices communicate with the Si7021 using a command structure. The commands are listed in the I2C
command table. Commands other than those documented below are undefined and should not be sent to the
device.
Table 10. I2C Slave Address Byte
A6 A5 A4 A3 A2 A1 A0 R/W
10000000
Table 11. I2C Command Table
Command Description Command Code
Measure Relative Humidity, Hold Master Mode 0xE5
Measure Relative Humidity, No Hold Master Mode 0xF5
Measure Temperature, Hold Master Mode 0xE3
Measure Temperature, No Hold Master Mode 0xF3
Read Temperature Value from Previous RH Measurement 0xE0
Reset 0xFE
Write RH/T User Register 1 0xE6
Read RH/T User Register 1 0xE7
Read Electronic ID 1st Byte 0xFA 0x0F
Read Electronic ID 2nd Byte 0xFC 0xC9
Read Firmware Revision 0x84 0xB8
Si7021-A10
Rev. 1.0 19
5.1. Issuing a Measurement Command
The measurement commands instruct the Si7021 to perform one of two possible measurements; Relative Humidity
or Temperature. The procedure to issue any one of these commands is identical. While the measurement is in
progress, the option of either clock stretching (Hold Master Mode) or Not Acknowledging read requests (No Hold
Master Mode) is available to indicate to the master that the measurement is in progress; the chosen command
code determines which mode is used.
Optionally, a checksum byte can be returned from the slave for use in checking for transmission errors. The
checksum byte will follow the least significant measurement byte if it is acknowledged by the master. The
checksum byte is not returned if the master “not acknowledges” the least significant measurement byte. The
checksum byte is calculated using a CRC generator polynomial of x8 + x5 + x4 + 1, with an initialization of 0x00.
The checksum byte is optional after initiating an RH or temperature measurement with commands 0xE5, 0xF5,
0xE3, and 0xF3. The checksum byte is required for reading the electronic ID with commands 0xFA 0x0F and 0xFC
0xC9. For all other commands, the checksum byte is not supported.
Master Slave
Sequencetoperformameasurementandreadbackresult(HoldMasterMode)
SSlave
AddressWAMeasure
CmdASrSlave
AddressRA
Clockstretch
during
measurement

MSByteALSByteNAP

AChecksumNA P
Si7021-A10
20 Rev. 1.0
Si7021-A10
Rev. 1.0 21
5.1.1. Measuring Relative Humidity
Once a relative humidity measurement has been made, the results of the measurement may be converted to
percent relative humidity by using the following expression:
Where:
%RH is the measured relative humidity value in %RH
RH_Code is the 16-bit word returned by the Si7021
A humidity measurement will always return XXXXXX10 in the LSB field.
Due to normal variations in RH accuracy of the device as described in Table 4, it is possible for the measured value
of %RH to be slightly less than 0 when the actual RH level is close to or equal to 0. Similarly, the measured value
of %RH may be slightly greater than 100 when the actual RH level is close to or equal to 100. This is expected
behavior, and it is acceptable to limit the range of RH results to 0 to 100%RH in the host software by truncating
values that are slightly outside of this range.
5.1.2. Measuring Temperature
Each time a relative humidity measurement is made a temperature measurement is also made for the purposes of
temperature compensation of the relative humidity measurement. If the temperature value is required, it can be
read using command 0xE0; this avoids having to perform a second temperature measurement. The measure
temperature commands 0xE3 and 0xF3 will perform a temperature measurement and return the measurement
value, command 0xE0 does not perform a measurement but returns the temperature value measured during the
relative humidity measurement.
The checksum output is not available with the 0xE0 command.
%RH 125RH_Code
65536
--------------------------------------- 6–=
SequencetoreadtemperaturevaluefrompreviousRHmeasurement
SSlave
AddressWA0xE0ASrSlave
AddressRAMSByte
ALSByteNAP
Si7021-A10
22 Rev. 1.0
The results of the temperature measurement may be converted to temperature in degrees Celsius (°C) using the
following expression:
Where:
Temperature (°C) is the measured temperature value in °C
Temp_Code is the 16-bit word returned by the Si7021
A temperature measurement will always return XXXXXX00 in the LSB field.
Temperature (C175.72Temp_Code
65536
--------------------------------------------------------46.85–=
Si7021-A10
Rev. 1.0 23
5.2. Reading and Writing User Registers
There is one user register on the Si7021 that allows the user to set the configuration of the Si7021. The procedure
for accessing that register is described below.
The checksum byte is not supported after reading a user register.
5.3. Electronic Serial Number
The Si7021 provides a serial number individualized for each device that can be read via the I2C serial interface.
Two I2C commands are required to access the device memory and retrieve the complete serial number. The
command sequence, and format of the serial number response is described in the figure below:
First access:
Sequencetoreadaregister
SSlave
Address WAReadReg
Cmd ASr
Slave
Address RAReadData NA P
Sequencetowritearegister
SSlaveAddress W AWriteRegCmd AWriteData AP
Master Slave
SSlaveAddress W ACK 0xFA ACK 0X0F ACK
SSlaveAddress R ACK
SNA_3 ACK CRC ACK SNA_2 ACK CRC ACK
SNA_1 ACK CRC ACK SNA_0 ACK CRC NACK P
2nd access:
SSlaveAddress W ACK 0xFC ACK 0XC9 ACK
SSlaveAddress R ACK
SNB_3 ACK SNB_2 ACK CRC ACK
SNB_1 ACK SNB_0 ACK CRC NACK P
Si7021-A10
24 Rev. 1.0
The format of the complete serial number is 64-bits in length, divided into 8 data bytes. The complete serial number
sequence is shown below:
The SNB3 field contains the device identification to distinguish between the different Silicon Labs relative humidity
and temperature devices. The value of this field maps to the following devices according to this table:
0x00 or 0xFF engineering samples
0x0D=13=Si7013
0x14=20=Si7020
0x15=21=Si7021
5.4. Firmware Revision
The internal firmware revision can be read with the following I2C transaction:
The values in this field are encoded as follows:
0xFF=Firmware version 1.0
5.5. Heater
The Si7021 contains an integrated resistive heating element that may be used to raise the temperature of the
sensor. This element can be used to test the sensor, to drive off condensation, or to implement dew-point
measurement when the Si7021 is used in conjunction with a separate temperature sensor such as another Si7021
(the heater will raise the temperature of the internal temperature sensor).
The heater can be activated using HTRE, bit 2 in User Register 1. Turning on the heater will reduce the tendency
of the humidity sensor to accumulate an offset due to "memory" of sustained high humidity conditions.
SNA_3 SNA_2 SNA_1 SNA_0 SNB_3 SNB_2 SNB_1 SNB_0
SSlave
Address WA0x84 A0xB8A S Slave
Address
RAFWREV ANA P
Si7021-A10
Rev. 1.0 25
6. Control Register
6.1. Register Descriptions
Reset Settings = 0011_1010
Table 12. Register Summary
Register Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
User Register 1 RES1 VDDS RSVD RSVD RSVD HTRE RSVD RES0
Notes:
1. Any register not listed here is reserved and must not be written. The result of a read operation on these bits is
undefined.
2. Except where noted, reserved register bits will always read back as “1,” and are not affected by write operations. For
future compatibiliy, it is recommended that prior to a write operation, registers should be read. Then the values read
from the RSVD bits should be written back unchanged during the write operation.
Register 1. User Register 1
Bit D7 D6 D5 D4D3D2 D1 D0
Name RES1 VDDS RSVD RSVD RSVD HTRE RSVD RES0
Type R/W R R/W R/W R/W R/W R/W
Bit Name Function
D7; D0 RES[1:0] Measurement Resolution:
RH Temp
00: 12 bit 14 bit
01: 8 bit 12 bit
10: 10 bit 13 bit
11: 11 bit 11 bit
D6 VDDS VDD Status:
0: VDD OK
1: VDD Low
The minimum recommended operating voltage is 1.9 V. A transi-
tion of the VDD status bit from 0 to 1 indicates that VDD is
between 1.8 V and 1.9 V. If the VDD drops below 1.8 V, the
device will no longer operate correctly.
D5, D4, D3 RSVD Reserved
D2 HTRE 1=On-chip Heater Enable
0=On-chip Heater Disable
D1 RSVD Reserved
Si7021-A10
26 Rev. 1.0
7. Pin Descriptions: Si7021 (Top View)
Pin Name Pin # Pin Description
SDA 1 I2C data
GND 2 Ground. This pin is connected to ground on the circuit board through a trace. Do not
connect directly to GND plane.
VDD 5 Power. This pin is connected to power on the circuit board.
SCL 6 I2C clock
DNC 3,4 These pins should be soldered to pads on the PCB for mechanical stability; they can be
electrically floating or tied to VDD (do not tie to GND).
TGND Paddle This pad is connected to GND internally. This pad is the main thermal input to the on-
chip temperature sensor. The paddle should be soldered to a floating pad.
DNC
SCL
VDD
1
2
6
5GND
SDA
3 4
DNC
Si7021-A10
Rev. 1.0 27
8. Ordering Guide
Table 13. Device Ordering Guide
P/N Description Max. Accuracy Pkg Operating
Range (°C) Protective
Cover Packing
Format
Temp RH
Si7021-A10-GM Digital temperature/ humidity sensor ±0.4 °C ± 3% DFN 6 –40 to +85 °C N Tube
Si7021-A10-GMR Digital temperature/ humidity sensor ±0.4 °C ± 3% DFN 6 –40 to +85 °C N Tape &
Reel
Si7021-A10-GM1 Digital temperature/ humidity sensor ±0.4 °C ± 3% DFN 6 –40 to +85 °C Y Cut Tape
Si7021-A10-
GM1R
Digital temperature/ humidity sensor ±0.4 °C ± 3% DFN 6 –40 to +85 °C Y Tape &
Reel
Si7021-A10-IM Digital temperature/ humidity sensor –
industrial temp range
±0.4 °C ± 3% DFN 6 –40 to +125 °C N Tube
Si7021-A10-IMR Digital temperature/ humidity sensor –
industrial temp range
±0.4 °C ± 3% DFN 6 –40 to +125 °C N Tape &
Reel
Si7021-A10-IM1 Digital temperature/ humidity sensor –
industrial temp range
±0.4 °C ± 3% DFN 6 –40 to +125 °C Y Cut Tape
Si7021-A10-IM1R Digital temperature/ humidity sensor –
industrial temp range
±0.4 °C ± 3% DFN 6 –40 to +125 °C Y Tape &
Reel
Si7021-A10-YM0 Digital temperature/ humidity sensor –
automotive
±0.4 °C ± 3% DFN 6 –40 to +125 °C N Tube
Si7021-A10-YM0R Digital temperature/ humidity sensor –
automotive
±0.4 °C ± 3% DFN 6 –40 to +125 °C N Tape &
Reel
Si7021-A10-YM1 Digital temperature/ humidity sensor –
automotive
±0.4 °C ± 3% DFN 6 –40 to +125 °C Y Cut Tape
Si7021-A10-YM1R Digital temperature/ humidity sensor –
automotive
±0.4 °C ± 3% DFN 6 –40 to +125 °C Y Tape &
Reel
Note: The “A” denotes product revision A and “10” denotes firmware version 1.0.
Si7021-A10
28 Rev. 1.0
9. Package Outline
9.1. Package Outline: 3x3 6-pin DFN
Figure 10. 3x3 6-pin DFN
Table 14. 3x3 6-pin DFN Package Diagram Dimensions
Dimension Min Nom Max
A 0.70 0.75 0.80
A1 0.00 0.02 0.05
b 0.35 0.40 0.45
D 3.00 BSC.
D2 1.40 1.50 1.60
e 1.00 BSC.
E 3.00 BSC.
E2 2.30 2.40 2.50
H1 0.85 0.90 0.95
H2 1.39 1.44 1.49
L 0.35 0.40 0.45
aaa 0.10
bbb 0.10
ccc 0.05
ddd 0.10
eee 0.05
fff 0.05
Notes:
1. All dimensions shown are in millimeters (mm).
2. Dimensioning and Tolerancing per ANSI Y14.5M-1994.
Si7021-A10
Rev. 1.0 29
9.2. Package Outline: 3x3 6-pin DFN with Protective Cover
Figure 8 illustrates the package details for the Si7021 with the optional protective cover. The table below lists the
values for the dimensions shown in the illustration.
Figure 8. 3x3 6-pin DFN with Protective Cover
Table 15. 3x3 6-pin DFN with Protective Cover Package Diagram Dimensions
Dimension Min Nom Max
A——1.21
A1 0.00 0.02 0.05
A2 0.70 0.75 0.80
b 0.35 0.40 0.45
D 3.00 BSC.
D2 1.40 1.50 1.60
e 1.00 BSC.
E 3.00 BSC.
E2 2.30 2.40 2.50
F1 2.80 2.85 2.90
F2 2.80 2.85 2.90
h 0.76 0.83 0.90
L 0.35 0.40 0.45
R1 0.45 0.50 0.55
aaa 0.10
bbb 0.10
ccc 0.05
ddd 0.10
eee 0.05
Notes:
1. All dimensions are shown in millimeters (mm).
2. Dimensioning and Tolerancing per ANSI Y14.5M-1994.
Si7021-A10
30 Rev. 1.0
10. PCB Land Pattern and Solder Mask Design
Figure 9. Si7021 PCB Land Pattern
Table 16. PCB Land Pattern Dimensions
Symbol mm
C1 2.90
E1.00
P1 1.60
P2 2.50
X1 0.45
Y1 0.85
Notes:
General
1. All dimensions shown are at Maximum Material Condition (MMC). Least Material
Condition (LMC) is calculated based on a Fabrication Allowance of 0.05 mm.
2. This Land Pattern Design is based on the IPC-7351 guidelines.
Solder Mask Design
3. All metal pads are to be non-solder mask defined (NSMD). Clearance between the
solder mask and the metal pad is to be 60 µm minimum, all the way around the
pad.
Stencil Design
4. A stainless steel, laser-cut and electro-polished stencil with trapezoidal walls
should be used to assure good solder paste release.
5. The stencil thickness should be 0.125 mm (5 mils).
6. The ratio of stencil aperture to land pad size should be 1:1 for all perimeter pins.
7. A 2x1 array of 1.00 mm square openings on 1.30 mm pitch should be used for the
center ground pad to achieve a target solder coverage of 50%.
Card Assembly
8. A No-Clean, Type-3 solder paste is recommended.
9. The recommended card reflow profile is per the JEDEC/IPC J-STD-020
specification for Small Body Components.
Si7021-A10
Rev. 1.0 31
11. Top Marking
11.1. Si7021 Top Marking
11.2. Top Marking Explanation
12. Additional Reference Resources
AN607: Si70xx Humidity Sensor Designer’s Guide
Mark Method: Laser
Font Size 0.30 mm
Pin 1 Indicator: Circle = 0.30 mm Diameter
Upper-Left Corner
Line 1 Marking: TTTT = Mfg Code
Si7021-A10
32 Rev. 1.0
DOCUMENT CHANGE LIST
Revision 0.1 to Revision 0.9
Updates to Section 1. Electrical Specifications.
Updated Table 2. General Specifications.
Update Figure 1. I2C Interface Timing Diagram.
Updated Section 2. Typical Applications Circuits.
Updated Table 6. Thermal Characteristics.
Updated Table 12. I2C Command Table.
Updated Section 4.4 PCB Assembly.
Updated Section 5.3. Measuring Relative Humidity.
Updated Section 5.4. Measuring Temperature.
Updated Section 5.5. Electronic Serial Number.
Updated Section 9.1. Package Outline.
Updated Section 11. Top Marking.
Revision 0.9 to Revision 0.95
Updated the Features on page 1
Updated block diagram
Added -I part numbers
Updated Table 1
Updated Table 2
Updated Table 3
Updated Table 4
Updated Table 5
Updated Figure 4
Updated Section 4.4.1.
Updated Table 12
Updated Section 5
Corrected a typo on Section 6.1, Register 1.
Updated firmware version to -A10 in Section 8
Revision 0.95 to Revision 1.0
Updated Table 2.
Clarified footnotes in Table 4.
Update Table 7.
Updated part numbers in Table 8.
Updated Section 5.1.
Updated Section 5.1.2.
Updated Section 5.4.
Updated footnotes in Table 12.
Updated description for Register 1, bit D6.
Updated automotive part numbers in Table 13.
Added firmware description to Table 13.
Si7021-A10
Rev. 1.0 33
CONTACT INFORMATION
Silicon Laboratories Inc.
400 West Cesar Chavez
Austin, TX 78701
Tel: 1+(512) 416-8500
Fax: 1+(512) 416-9669
Toll Free: 1+(877) 444-3032
Please visit the Silicon Labs Technical Support web page:
http://www.siliconlabs.com/support/pages/contacttechnicalsupport.aspx
and register to submit a technical support request.
Patent Notice
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Other products or brandnames mentioned herein are trademarks or registered trademarks of their respective holders.
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