SHT21 - 12 Pages

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Catalogue excerpts

Datasheet SHT21 Humidity and Temperature Sensor      Fully calibrated Digital output, I2C interface Low power consumption Excellent long term stability DFN type package – reflow solderable Product Summary SHT21, the new humidity and temperature sensor of Sensirion is about to set new standards in terms of size and intelligence: Embedded in a reflow solderable Dual Flat No leads (DFN) package of 3 x 3mm foot print and 1.1mm height it provides calibrated, linearized signals in digital, true I2C format. With a completely new designed CMOSens® chip, a reworked capacitive type humidity sensor and an improved band gap temperature sensor the performance has been lifted even beyond the outstanding level of the previous sensor generation (SHT1x and SHT7x). For example, measures have been taken to stabilize the behavior at high humidity levels. Dimensions Every sensor is individually calibrated and tested. Lot identification is printed on the sensor and an electronic identification code is stored on the chip – which can be read out by command. Furthermore, the resolution of SHT21 can be changed by command (8/12bit up to 12/14bit for RH/T), low battery can be detected and a checksum helps to improve communication reliability. With made improvements and the miniaturization of the sensor the performance-to-price ratio has been improved – and eventually, any device should benefit from the cutting edge energy saving operation mode. For testing SHT21 a new evaluation Kit EK-H4 is available. SHT21 features a generation 4C CMOSens® chip. Besides the capacitive relative humidity sensor and the band gap temperature sensor, the chip contains an amplifier, A/D converter, OTP memory and a digital processing unit. Material Contents While the sensor itself is made of Silicon the sensors’ housing consists of a plated Cu lead-frame and green epoxy-based mold compound. The device is fully RoHS and WEEE compliant, e.g. free of Pb, Cd and Hg. Additional Information and Evaluation Kits Figure 1: Drawing of SHT21 sensor package, dimensions are given in mm (1mm = 0.039inch), tolerances are ±0.1mm. NC and die pad (center pad) are internally connected to VSS. They may be left floating. VSS = GND, SDA = DATA. Numbering of E/O pads starts at lower right corner (indicated by notch in die pad) and goes clockwise (compare Table 2). Additional information such as Application Notes is available from the web page For more information please contact Sensirion via For SHT21 two Evaluation Kits are available: EK-H4, a four-channel device with Viewer Software, that also serves for data-logging, and a simple EK-H5 directly connecting one sensor via USB port to a computer.

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Sensor Performance Relative Humidity This datasheet is subject to change and may be amended without prior notice. 1 Default measurement resolution is 14bit (temperature) / 12bit (humidity). It can be reduced to 12/8bit, 11/11 bit or 13/10bit by command to user register. 2 Accuracies are tested at Outgoing Quality Control at 25°C (77°F) and 3.0V. Values exclude hysteresis and long term drift and are applicable to noncondensing environments only. 3 Time for achieving 63% of a step function, valid at 25°C and 1m/s airflow. 4 Normal operating range: 0-80%RH, beyond this limit sensor may read a reversible...

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Users Guide SHT2x For details on how Sensirion is specifying and testing accuracy performance please consult Application Note “Statement on Sensor Specification”. Relative Humidity (%) 1.1 Operating Range The sensor works stable within recommended Normal Range – see Figure 4. Long term exposure to conditions outside Normal Range, especially at humidity >80%RH, may temporarily offset the RH signal (+3%RH after 60h). After return into the Normal Range it will slowly return towards calibration state by itself. See Section 2.3 “Reconditioning Procedure” for eliminating the offset. Prolonged exposure...

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2.1 Soldering Instructions The DFN’s die pad (centre pad) and perimeter I/O pads are fabricated from a planar copper lead-frame by overmolding leaving the die pad and I/O pads exposed for mechanical and electrical connection. Both the I/O pads and die pad should be soldered to the PCB. In order to prevent oxidation and optimize soldering, the bottom side of the sensor pads is plated with Ni/Pd/Au. On the PCB the I/O lands8 should be 0.2mm longer than the package I/O pads. Inward corners may be rounded to match the I/O pad shape. The I/O land width should match the DFN-package I/O-pads width 1:1...

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2.2 Storage Conditions and Handling Instructions Moisture Sensitivity Level (MSL) is 2, according to IPC/JEDEC J-STD-020D.1; hence storage time is limited to one year after date of delivery. It is of great importance to understand that a humidity sensor is not a normal electronic component and needs to be handled with care. Chemical vapors at high concentration in combination with long exposure times may offset the sensor reading. For this reason it is recommended to store the sensors in original packaging including the sealed ESD bag at following conditions: Temperature shall be in the range...

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Table 4 DC characteristics of digital input/output pads. VDD = 2.1V to 3.6V, T = -40°C to 125°C, unless otherwise noted.

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commands from the master (MCU). Current consumption during start up is 350jA maximum. 5.2 Start / Stop Sequence Each transmission sequence begins with Start condition (S) and ends with Stop condition (P) as displayed in Figure 13 and Figure 14. Table 5 Timing specifications of digital input/output pads for I2C fast mode. Entities are displayed in Figure 12. VDD = 2.1V to 3.6V, T = -40°C to 125°C, unless otherwise noted. 5 Communication with Sensor SHT21 communicates with true I2C protocol. For information on I2C beyond the information in the following Sections please refer to the following website: Please...

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