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I'm going to use a DC outputting 10k windvane and the manufacturer recommends it to use it with a buffer with some additional resistors. This is the paper where they recommend how to condition the vane signal.

I want to power the vane with 5V so I decided use a rail-to-rail opamp LMC6482. They recomemnd an RC low pass filter after 1k so I just added C1 1u to form a low pass filter with 160Hz cut off. I will also add caps for the power rail of the opamp. Here is how I want to implement the circuit:

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Is my way of buffering and the opamp type is correct in my dcircuit diagram? Is the cut off freq. good for this type of application.

Should I use an isolation amplifier as an opamp? I would like to isolate vane and the output. Can I use a HCPL-7800 ?

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  • \$\begingroup\$ Isolation is for safety? if the vane and circuit is let's say epoxy encapsulated, then no danger. An opto-amp like HCPL would be needed if you have connection to the field and possibly hazardous voltages. One remark: since 50 and 60 Hz are everywhere and you are interested in dc, reduce the cutoff frequency to e.g. 10 Hz: it does not make wonders, but comes for free. \$\endgroup\$ – andrea Sep 4 '17 at 16:35
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The cutoff frequency of your filter is dependent on the input value of the vane, because it forms part of the source impedance of your circuit. If you want a well controlled cutoff frequency, put the filter after the amp.

I would also use a much bigger capacitor -- 100 uF/16V or higher are cheap and plentiful, and I don't think you have a space problem where an electrolytic cap would be in the way?

Btw, if the weathervane source impedance is 5k, and the filter resistance value is 1.1 kOhm, and you use a 1 uF capacitor, the RC time constant is 6.1 ms (about 1/164 second) but the filter frequency is about 26 Hz. However, when the vane is at the top, the source impedance is 1.2 kOhm, and at the bottom, it's 1.1 kOhm and the frequency adjusts accordingly (Just draw out the resistances as a ladder and this should become clear.)

Finally, if you're worried about high voltage build-up from static charge and from "random outdoors electromagnetics" when running a long cable, you could add a Zener or other TVS across the input terminals. That won't help if you get hit by lightning, but there are many smaller scale problems that can be avoided by doing so.

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  • \$\begingroup\$ Why do they recommend to buffer? The input impedance of the DAQ is very high already. \$\endgroup\$ – user1245 Sep 7 '17 at 10:25

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