SI7022-A20 Datasheet, PDF (16/25 Pages) Silicon Laboratories – HUMIDITY/TEMPERATURE SENSOR WITH PWM OUTPUT

Si7022-A20
5. PWM Output
During operation, the Si7022 takes a relative humidity and temperature measurement once per second and
converts the result into a pulse width modulated waveform. Two output pins are available for reading the
measurement results: PWM1 and PWM2. The information output on each pin is determined by the SELECT pin as
follows:
Table 10. PWM Outputs
Voltage on SELECT Pin
High
Low
PWM1 Output
RH
Temperature
PWM2 Output
Temperature
RH
The duty cycle of the waveform on the PWM outputs corresponds to the relative humidity and temperature results
shown in the following equations. The PWM duty cycle varies linearly with the measurement result between the
minimum and maximum values:
RH = – 6 + 125  DC_PWM
Equation 1.
Temperature = – 46.85 + 175.72  DC_PWM
Equation 2.
In the typical application, the PWM output is filtered using an RC network to provide an analog output voltage that
varies linearly with RH and temperature. See Section “2. Typical Application Circuits” for the recommended
application circuit.
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.
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Rev. 1.1