SHT20S datasheet(6/10 Pages) ETC2

6 / 10 page

Datasheet SHT20S

www.sensirion.com

Version 4 – May 2014

6/10

Parameter

min

max

Units

VDD to VSS

-0.3

Digital I/O Pins (SDA, SCL)

to VSS

-0.3

VDD + 0.3

Input Current on any Pin

-100

100

Table 3 Electrical absolute maximum ratings

ESD immunity is qualified according to JEDEC JESD22-

A114 method (Human Body Model at

4kV), JEDEC

JESD22-A115 method (Machine Model

200V) and ESDA

ESD-STM5.3.1-1999 and AEC-Q100-011 (Charged

Device Model, 750V corner pins, 500V other pins). Latch-

up immunity is provided at a force current of

100mA with

Tamb = 125°C according to JEDEC JESD78. For exposure

beyond named limits the sensor needs additional

protection circuit.

4.2

Input / Output Characteristics

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

min

typ

max Units

Output Low Voltage, VOL

Output High Voltage, VOH

VDD

Output Sink Current, IOL

µA

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

Start up Sensor

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

SDM Output Principle

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.

Figure 10 Schematic principle of SDM signal. X represents

either RH or T at different levels of sensor output.

5.3

Converting SDM to Analog Signal

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:

RLP = 100kΩ and CLP = 220nF. By pulling SCL low or high, the

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 RLP. Furthermore, the current should be

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.

SDA

VDD

SDA

VDD

SDA

VDD

X ≈ 10%

X ≈ 50%

X ≈ 90%

SDA

SCL

VSS

VDD

SHT20S

VSO

Select RH or T

RLP

CLP

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