Many of our new data acquisition modules (TM #128, IOM #142 and others in the future) provide an interface with the Dallas DS2438Z sensor which provides a temperature measurement and two 10-bit A/D converters, one on V_dd and the other on terminal V_ad.
This might be used to measure relative humidity using the . This device is in a three terminal SIP with leads on 0.1 inch centers.
The output of the HIH-3610-001 is linearly related to relative humidity. By connecting V_out of the HIH3610 to the V_ad input of the DS2438Z and powering the HIH-3610 using the same supply as the that which powers the DS2438;
V_ad = V_dd * (0.0062 * RH_raw + 0.16)This may be arranged;
RH_raw = 161.29 * V_ad / V_dd - 25.80 RH_true = RH_raw / (1.0546 - 0.00216 * Tc)
As in my discussion of the data acquisition modules, I stronlgy discourage "stealing power" from the DQ data lead to power the DS2438 and HIH-3610 sensors. Each HIH-3610 may draw up to 200 uA, and this results in a full 1.0 volt drop on the DQ lead. Add another, the +5 VDC on the DQ lead is as low as 3.0 VDC and chances are the entire network of Dallas 1-W devices will fail.
Rather, run +5 VDC and GRD on a separate twisted pair from the data acqusisition module to each of the DS2438 sensors. This permits you to operate up to 16 DS2438 devices onthe same run without having to worry about pulling the DQ data lead so low that the controller is unable to communicate with any devices on the run. CAT 5 cable is cheap and contains four pairs. Use one pair for the DQ data lead and ground and another for +5 VDC and GRD.