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Frequently asked Questions Humidity Sensors SHT11/SHT15; SHT71/SHT75
Technology Humidity Sensors SHT1x/SHT7x
1. What is CMOSens?
2. What is a Sensmitter?
Reliability / Quality humidity sensor SHT1x/SHT7x
3. What kind of reliability test did you do?
4. Will the humidity sensor SHT1x/SHT7x be destroyed if it is operated outside the indicated temperature range?
5. Is your bi-directional 2-wire interface compatible with I2C?
6. What is the minimal speed on the SCK serial clock?
7. How many meters of cable can be connected to a humidity sensor SHT1x/SHT7x?
8. I can’t get the serial communication to work properly. What should I do?
9. Is it possible to reduce the standby current (<0.1uA)?
Humidity Sensor SHT1x/7x
10. What is the measurement principle of the humidity sensor SHT1x/SHT7x?
11. How can I effectively protect the SHTxx humidity sensor against pollution?
12. What happens in the case of condensation on the SHTxx humidity sensor?
13. What happens in the case of complete submersion of the humidity sensor?
14. Should the SHTxx humidity sensors be cleaned?
15. The humidity reads too low/high is the humidity sensor SHTxx bad?
16. What happens to the accuracy/output if I use this humidity sensor at high pressure (>50 psi)?
Temperature Sensor of the humidity sensor SHT1x/SHT7x
17. What is the measurement principle of the temperature sensor?
18. Is the temperature sensor a distinct second sensor system or is it combined with the relative humidity sensor?
19. The temperature reads too low/high is the SHTxx humidity sensor bad?
Mounting humidity sensor SHT1x/SHT7x
20. Why can’t I route signal lines under the humidity sensor SHT1x in the area marked "no copper" on the datasheet?
Soldering humidity sensor SHT1x/SHT7x
21. Is there a through-hole version available for the SHT1x humidity sensor?
Troubleshooting
22. SHTxx humidity sensor shows too little RH?
23. Clean POWER ON of the SHTxx
1. What is CMOSens?
CMOSens® (Ce-mo-sens) is a new, proprietary base technology that sets the scale for the highest precision sensor systems. The fusion of semiconductor chips (CMOS) and sensor technology produces highly integrated system solutions which are impressive because of excellent sensor precision, as well as digital intelligence and reliability. In CMOSens®, the sensor element, amplifier, and A/D converter form one unit on the same silicon chip. The digital intelligence associated with the CMOSens® sensor allows the output of a completely calibrated, temperature-compensated output signal. The CMOSens® "Intelligence" integrated on the chip enables a conceptually simple processing of the output measurement data by means of standard digital interfaces, such as, e.g., SPI.
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2. What is a Sensmitter ?
Sensmitter stands for the combination of "sensor" and "transmitter". Eventhough the humidity sensor SHTxx is not a transmitter but rather a sensor, its innovative CMOSens® technology combines the functions of a traditional humidity sensor element with the ease-of-use and functionality of a transmitter (A/D conversion, digital 2-wire output, calibration memory). Thus, the SHTxx humidity sensor allows cost to be saved due to faster and easier system integration. In addition, the SHTxx humidity sensor provide - unlike other sensor elements - a combined relative humidity and temperature measurement, which is of particular interest for precise dew point measurement.
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3. What kind of reliability test did you ?
The SHT1x/SHT7x humidity sensors successfully passed many extreme endurance and accelerated ageing tests. Please contact Sensirion for more information.
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4. Will the SHT1x/SHT7x humidity sensor be destroyed if it is operated outside the indicated temperature range ?
Short time exposure to high temperatures(<280°C) will not damage the humidity sensor. However extreme temperatures accelerate ageing and may cause a (usually reversible) shift in the humidity sensing signal.
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5. Is your bi-directional 2-wire interface compatible with I2C ?
No it is not. The difference is in the start/stop sequence (transition start with SHTxx) and how the SHTxx humidity sensor signals the end of a measurement. SHTxx should not share a I2C bus with I2C components. However sharing of the SCK line and using a dedicated DATA line for the SHTxx humidity sensor will work well.
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6. What is the minimal speed on the SCK serial clock ?
There is no minimal speed. The SHTxx humidity sensor contains fully static logic and can be operated at very low frequencies.
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7. How many meters of cable can be connected to a SHT1x/SHT7x humidity sensor ?
The humidity sensor SHTxx devices are exceptionally well suited to retain reliable functionality in demanding environments. Advantages are listed below:
No external components (except the pull up resistor on the DATA line) are required. All susceptible circuit is integrated at micrometer scale making it inherently immune against RF radiation (originated by wireless devices). The pure digital interfacing to the humidity sensor SHTxx devices allows low S/N ratio on the SCK and DATA lines.
The transmission frequency of the measurement data through the SCK and DATA lines can be set arbitrarily low hence the slopes of the signals can be filtered to minimize susceptibility against bursts.
The CRC (cyclic redundancy check) allows the user to verify whether the transfer of the data has been performed properly. For more details refer to the application note "CRC".
SENSIRION recommends the following basic precautions when designing with the humidity sensor SHT11 device and long cables:
Using a capacitor between VDD and GND as close as possible to the humidity sensor SHTxx pins
Avoid running the SCK and DATA lines next to each other when using a long ribbon cable or flat flexible cable to minimize cross talk.
Reduce transmission frequency and slopes of the signals when using long cables. This is to reduce cross talk sensitivity, reflections and susceptibility against bursts. Slopes can be reduced by inserting a passive low pass filter, shown in Fig. 5. Best results may be achieved when using a low pass filter on either side of the cable connection
Please consult the application note "ESD, Latch-UP and EMC" for further information.
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8. I can’t get the serial communication to work properly. What should I do ?
Check the SCK and DATA line with an oscilloscope. Is SCK going from GND to VDD and back completely? Is DATA only changing at the negative edge of SCK? If no, modify your software. Is there any crosstalk between SCK and DATA? If yes separate the lines or reduce slew rates.
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9. Is it possible to reduce the standby current (<0.1uA) ?
The standby current is the leakage current of the transistors in the SHTxx humidity sensor. It can not be further reduced. However cutting off the power supply using a outside transistor or even connecting the power supply to a digital pin of a microcontroller is possible. Startup times must be observed.
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10. What is the measurement principle of the humidity sensor SHT1x/SHT7x?
Humidity is measured through change of capacitance in a special sensing polymer. This change is measured with a 14bit A/D converter and calibrated on chip.
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11. How can I effectively protect the SHTxx humidity sensor against pollution ?
The standard package protects the SHTxx humidity sensor well from various environments (Sunlight, touching,…) additional protection depending on the environment may be added around the SHTxx humidity sensor. Metal caps or membranes are often used.
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12. What happens in the case of condensation on the humidity sensor ?
While exposed to condensation the SHTxx humidity sensor measure at ~100%. They recover immediately when condensation is removed. Dew drops in and on the package will delay recovery.
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13. What happens in the case of complete submersion of the humidity sensor?
If the sensing element inside the package is completely covered with water, the SHTxx humidity sensor may temporarily measure below 90%. Normal operation resumes immediately after the water is removed.
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14. Should the SHTxx humidity sensors be cleaned ?
Cleaning is not required except if dust accumulates in the humidity sensor opening.
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15. The humidity reads too low/high is the SHTxx humidity sensor bad ?
Did you soak the SHTxx humidity sensor in high humidty after soldering as described in the application note? Is your reference recently calibrated? Is your test chamber in thermodynamic equilibrium? No temperature or humidity gradient or drift.
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16. What happens to the accuracy/output if I use SHT11 humidity sensors at high pressures (>50 psi) ?
The SHTxx Series humidity sensors are not affected by pressure changes.
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17. What is the measurement principle of the temperature sensor ?
Temperature is measured with a bandgap circuit by producing a VPTAT (=Voltage Proportional To Temperature). It is then A/D converted at 12 or 14 bits.
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18. Is the temperature sensor a distinct second sensor system or is it combined with the relative humidity sensor ?
Temperature is measured at the same location as humidity. This leads to excellent performance when calculating the dewpoint temperature since both sensors share the same substrate and react the same.
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19. The temperature reads too low/high is the SHTxx humidity sensor bad ?
Check VDD and GND with an oscilloscope
Add a 100 nf capacitor between them.
Is your test chamber in thermodynamic equilibrium? No temperature / humidity gradient or dirft?
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20. Why can’t I route signal lines under the SHT1x humidity sensor in the area marked "no copper" on the datasheet ?
The bottom of the SHT1x PCB is used for routing and should therefore not have contact to any conducting surface. If you are sure that the solder resist on your PCB will prevent any contact to the SHT1x humidity sensor you may route signals under the SHT1x at your own risk.
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21. Is there a through-hole version available for the SHT1x humidity sensor ?
A through hole version is available as samples. It is called SHT7x. Production is scheduled to start in summer 2002.
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22. SHTxx humidity sensor shows too little RH ?
Compare the temperature to calibrated reference. The humidity sensor may be warmer than environment. Check for heat sources. Remove anything causing a temperature gradient between the humidity sensor and the air to be measured.
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23. Clean POWER ON of the SHTxx
It is important that the SHTxx be powered up cleanly or it may lockup and not respond to commands. This can be caused by a combination of three factors:
1. A possibility that voltage at VDD rises very slowly during power up (typically VDD rises slower than within 50 ms to 90% of its target value).
2. Spikes are present on the power supply during the power-up phase.
3. SCK is floating or held high during power up.
To ensure that power-up is clean, the following precautions should be taken:
1. Power supply pins (VDD, GND) should be decoupled with a .01uF capacitor to improve the supply voltage integrity during the power-up phase.
2. If it cannot be guaranteed that the SCK line is low on power up, add a 10k pulldown resistor .
Also consider using an output pin of the microprocessor to supply Vdd to the SHTxx (this pin must be capable of providing approximately 0.5 mA peak). Using such an output pin allows toggling of the power to the SHTxx in case locking occurs. The firmware can then be written to detect the lack of proper communication and retry as necessary. The lockup only happens on power up - once the SHTxx is powered up and communicating properly the lockup will not occur.
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