Hello
I plan on using 3 temperature sensors (ds18b20) in paralel.
My doubt is, does it require a capacitor?
Hi,
It would be advisable to put a 0.1uF capacitor across the Vdd and GND wires to bypass any processor or external noise.
How long is the string of 18B20's?
Tom.. 
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No, but a capacitor from Vdd to GND may help, and won't hurt.
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1 meter long.
Why?
And another question: what kind of capacitor?
Hi,
The longer the sensor wiring the more it can act like an antenna an pick up noise and interference.
0.1uF ceramic or monolithic will get you some nice small capacitors.
Tom...
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I'd add a 100nF decoupling right at each sensor (it won't do much good anywhere else). I've used a string of 20 of these sensors this way 24/7 with complete reliability. When you have a long cable visiting multiple sensors there's plenty of chance for noise and interference to get into the cable so decoupling seems wise.
I'd definitely not recommend using parasite power mode with a long string of these sensors for the same reason.
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does it make sense like this?
Yup
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