Capacitive Noise in Thermocouple Interface Circuit Board Reverse Engineering

Capacitive Noise in Thermocouple Interface Circuit Board Reverse Engineering is an inevitable subject, it is evident that the noise voltage, VCOUPLED appearing across Z1, may be reduced by several means, all of which reduce noise current in Z1.

There is a capacitance between any two conductors separated by a dielectric (air or vacuum are dielectrics). If there is a change of voltage on one, there will be a movement of charge on the other. A basic model for this is shown in below Figure.

Capacitive Coupling Equivalent Circuit Model

They are reduction of the signal voltage VN, reduction of the frequency involved, reduction of the capacitance, or reduction of Z1 itself. Unfortunately however, often none of these circuit parameters can be freely changed, and an alternate method is needed to minimize the interference.

The best solution towards reducing the noise coupling effect of C is to insert a grounded conductor, also known as a Faraday shield, between the noise source and the affected circuit by PCB Reverse Engineering. This has the desirable effect of reducing Z1 noise current, thus reducing VCOUPLED.

A Faraday shield model is shown by below Figure. In the left picture, the function of the shield is noted by how it effectively divides the coupling capacitance, C. In the right picture the net effect on the coupled voltage across Z1 is shown. Although the noise current IN still flows in the shield, most of it is now diverted away from Z1. As a result, the coupled noise voltage VCOUPLED across Z1 is reduced.