Datasheet

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16
EEH210
The physical value RH given above corresponds to the relative humidity above liquid water according to World
Meteorological Organization (WMO).
The temperature T is calculated by inserting the ratio of tPW and tF into the following formula (result in °C):
T=-46.85+175.72∙
t
PW
tF
8.2. PDm outPut
PDM signal is a pulse sequence that with a low pass filter may be converted into analog voltage output. The data signal
is provided on SDA line. By setting SEL[1:0] as ‘10’, the PDM output mode is selected. Humidity and temperature output
mode is selected by SCL level. The sensor measures twice per second. Output resolution of RH and Temperature are set
to 10bit and 12 bit each.
PDM output
Pulse Density Modulation is a bit-stream of pulses; the more high pulses the higher the value in the full measurement
range – see Figure 18.
Figure 18: Schematic principle of PDM signal. X represents either RH or T at different levels of sensor output.
Converting PDM to Analogue Signal
A PDM signal normally is converted to an analogue voltage signal by the addition of a low-pass filter. Figure 19 displays
a typical circuit where a simple RC-filter is used.
Figure 19: 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 7 Hz is recommended. Typical values
forthelowpassfiltercomponentsareR=100kΩandC=220nF.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40μA.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50MΩ(60nA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.