Wiring SHT-75 humidity sensor

I want to wire the Sensirion SHT-75 sensor with many meters of cable. (e.g. 10m) There are some discussions about this in the old forum, but I didn't find a conclusive answer.

Connection on sensorside: I have currently used: Female connector RM 1,27 2 x 4 poles 605-008-1-2-00 W & P Products (supplier: Conrad nr 739193) These are just a bit too small for the sensor, but i can get them to fit.. For the first test with 10cm cable I soldered flat cable to the pins.

For the long cable, I was looking into shielded cable. I looked at STP cable. (4 shielded twisted pairs)

This leaves me currently with the following questions: 1) Anyone has a better connector for connecting to the SHT-75 ? 2) The STP cable has 4 pairs. So 8 lines for 4 lines. Should I best use one cable of each pair for each line (Vdd, Gnd, SCK, DATA), or should I combine the Vdd and GND with the SCK and DATA line to have better results? 3) Would using this shielded cable avoid me having to use a filter setup between the arduino and the cable? (as mentioned in: http://arduino.cc/forum/index.php/topic,14183.0.html ) I understand correctly this should be on the sensor side of the cable?


There is no conclusive answer.

A twisted pair cable has certain characteristics, and if a dedicated signal is used, it is a very good way to communicate. A current through one wire of a pair will automatically introduce an opposite current in the other wire of the pair because they are magnetically coupled. A shielded cable will reduce the influence of electrical noise.

But you have digital signals, and using twised pair will introcude a capacitance between the paired wires.

So what is important here ? Well, that's easy to say: the DATA and SCK should not influence each other, and the Vdd and GND should be free of noise.

The nice thing about twisted pair is that one pair has very little influence on other pairs. So use one pair for DATA and GND. Another pair for SCK and GND. Use Vcc and GND for the third pair, because of the magnetic coupling (current from Vcc will flow back in other wire of same pair). And don't connect the fourth pair.

If you need more current, you could use the fourth pair. But do the same as in the third pair: Vcc and GND in the fourth pair. Vcc and GND must be paired together because they need to be magnetically coupled.

At the sensor a decoupling capacitance at Vcc and GND is needed. This is very often 100nF.

Because of the cable capacitance of SCK to ground and DATA to ground, you have to use slow signals.

Connect the shield of the cable on the Arduino side to ground, and don't connect it at the sensor end. Otherwise the ground current is disturbed.

You could have some protection on both end, or even an lc-filter. But with a shielded cable, you could try without.