Understanding reason for buffering in a recommended signal conditioning interface

Interfacing a sensor and buffering DC with isolation

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 10V or 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:



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 ?

I really want to isolate the windvane circuit input and output. What is the simplest way to do that? Is there an opamp or module?

Re: Interfacing a sensor and buffering DC with isolation

Hello doncarlosalbatros,

the filter design depends on the response time you need to have to catch if the wind direction changes. Since the variable resistor from the wind vane is already isolated, it makes normally not much sense to isolate the signal and you need to think about the supply voltage (battery or supply from a power supply outside?). Isolation makes sense if you have long signal lines in a noisy environment and want to avoid ground loops.

Enjoy your design work!

I will make an interface for a 10k wind-vane recommended in this document. The vane cable will be like 50m long. I will follow the recommendations but I don't understand the explanations of the document and I have at the end four short questions regarding the same interface which are confusing to me.

Here is the recommended way of interfacing the vane from the document:



I want to use 12V supply for the opamp buffer and around 8.5V(through a linear regulator) for the vane. So I wanted to simulate all these in LTspice where I mimic the potentiometer of the vane by varying R4 and R3 in the following circuit:



If my drawing to LTspice is correct(?):

The document says:

For most data acquisition systems, an amplifier is desirable to buffer the sensor signal from the any loading in the data acquisition system.

Click to expand...

First of all the document adds resistors to overcome deadband issues and shorts ect. and this introduces error. But the same document suggests buffering. Most of data acquisition systems have at least 100Meg input impedance which would not cause loading. What can be the benefit of buffering here? Im afraid if I use opamp buffer I might cause some EMI pickup?

No idea what you mean by "deadband issues" related to this circuit. I presume that the low-pass is intended to filter high frequency noise, e.g. radio signals picked up by the cable. The purpose of the 270k resistor isn't obvious. Primarily it generates a small sensor linearity error.

Input impedance of data acquisition systems can be quite different. Yes, if you have a system with > 1 Mohm input, the buffer can be omitted. But other people might use an ADC which requires a low source resistance in the kohm range. The answer really depends.