SHT21S SENSIRION, SHT21S Datasheet - Page 6

• Manufacturer Part Number: SHT21S
• Description: SENSOR, HUMIDITY & TEMP
• Manufacturer: SENSIRION

Specifications of SHT21S

• Humidity Range: 0 - 100% RH
• Sensor Output: SDM - Convertible To Analogue
• Sensor Terminals: SMD
• Response Time: 8s
• Supply Voltage Range: 2.1V To 3.6V
• Accuracy: 2% RH
• Supply Voltage: 3VDC
• Rohs Compliant: Yes
• Capacitance: 0µF
• Sensing Accuracy: 2%
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 2 - 1 Year

Datasheet SHT21S
Parameter
VDD to VSS
Digital I/O Pins (SDA, SCL)
to VSS
Input Current on any Pin
Table 3 Electrical absolute maximum ratings
ESD immunity is qualified according to JEDEC JESD22-
A114E method (Human Body Model at 4kV), JEDEC
JESD22-A115A method (Machine Model 200V) and
ESDA
(Charged Device Model, 750V corner pins, 500V other
pins). Latch-up immunity is provided at a force current of
100mA with T
For exposure beyond named limits the sensor needs
additional protection circuit.
4.2
The electrical characteristics such as power consumption,
low and high level input and output voltages depend on
the supply voltage. For proper communication with the
sensor it is essential to make sure that signal design is
strictly within the limits given in Table 4.
Parameter
Output Low Voltage, VOL
Output High Voltage, VOH
Output Sink Current, IOL
Table 4 DC characteristics of input / output pad. VDD = 2.1 V to
3.6 V, T = -40 °C to 125 °C, unless otherwise noted.
5 Communication with Sensor
5.1
As a first step, the sensor is powered up to the chosen
supply voltage VDD (between 2.1V and 3.6V). After
power-up, the sensor needs at most 150ms for reaching
idle state. During that time SDA is in undefined state. Then
the sensor starts measuring and providing data on SDM
bit-stream.
5.2
Sigma Delta Modulation is a bit-stream of pulses; the more
high pulses the higher the value in the full measurement
range – see Figure 10. Such information is humidity for
SCL pulled high and temperature for SCL pulled low. The
fundamental frequency of SDM is in the range of roughly 4
kHz and 65 kHz.
www.sensirion.com
Input / Output Characteristics
Start up Sensor
SDM Output Principal
ESD-STM5.3.1-1999
amb
= 125°C according to JEDEC JESD78.
-100
-0.3
-0.3
min
min
and
VDD + 0.3
VDD
max
100
typ
0
5
AEC-Q100-011
max Units
40
Units
mA
Version 1.1 – May 2010
V
V
µA
V
V
Figure 10 Schematic principle of SDM signal. X represents
either RH or T at different levels of sensor output.
5.3
An SDM signal normally is converted to an analog voltage
signal by the addition of a low-pass filter. Figure 11
displays a typical circuit where a simple RC-filter is used.
For conversion into physical values please read the
following Chapter.
Figure 11 Typical circuit with low pass filter (surrounded by
hatched line) for analog output. Recommended component size:
R
output value is switched to temperature or humidity,
respectively.
For an acceptable small ripple of the analog voltage
signal, a cut-off frequency of 7Hz is recommended. Typical
values for the low pass filter components are R = 100kΩ
and C = 220nF. The corresponding ripple of the signal is
limited to maximal amplitude of ±0.2%RH and ±0.28°C,
respectively. If larger deviations are acceptable the
capacitor size can be reduced.
Important: The maximum current from SDA should not
exceed 40µA. Therefore, there are restrictions on the size
of the resistance R
kept as low as possible and therefore the input impedance
of the reading buffer shall be larger than 50MΩ (60nA
input biased current). Eventually, cable length between
sensor and low pass filter shall be kept as short as
possible in order to prevent self heating.
Please note, that ripples and impacts by impedance are
not considered in the accuracy statement.
X ≈ 10%
X ≈ 50%
X ≈ 90%
LP
SDA
SDA
SDA
= 100kΩ and C
Converting SDM to Analog Signal
Select RH or T
V
SO
LP
LP
= 220nF. By pulling SCL low or high, the
. Furthermore, the current should be
C
LP
R
LP
VSS
VDD
SCL
SDA
SHT21S
VDD
VDD
VDD
6/9
0V
0V
0V