Mechanical analogies can be very useful for under standing electrical concepts.
Electricity, that unseen force that powers much of the world around us, is an abstract concept to most people because we can’t see it or touch it safely.
The simple mechanical system of a bicycle can provide understanding of a common electrical circuit - the differential input.
One of the best ways to avoid extraneous noise pickup in a sound reinforcement system is to properly utilize the differential input on each piece of equipment.
The differential input is sensitive to the instantaneous difference between the voltage that arrives at each input terminal.
Let’s use a bicycle to gain understanding about this concept.
A professional cyclist utilizes devices on the pedals that allow both the downward pressure and upward pres sure of the foot to be transmitted to the rear wheel.
Figure 1 shows a time domain plot of the position of each pedal. Note that this appears as a sine wave when viewed as a function of time, and that there is always a 180 degree phase differential between the two pedals at any instant.
Since this differential is fixed and independent of peddling frequency, we could say that the two pedals have a reverse-polarity relationship.
As one pedal is being pushed down by one foot, the other is being pulled up by the other foot. This type of relationship is aptly known as “push pull” and is common in many mechanical and electrical systems. The sprocket is turned by the differential pressure from the two pedals, which is twice the pressure of either pedal considered individually.
Also of interest is that if a positive or negative pressure were exerted on both petals simultaneously, no rotation would occur because these two pressures are of opposite polarity and will cancel out at the sprocket. Only when a differential pressure is exerted does the bike move.
We could say that our sprocket has high common-mode impedance, because pressures that are identical on both pedals do not cause the bike to move. It has low differential impedance, in that pressures that are reverse-polarity on each pedal cause the bike to go forward. The input impedance of the sprocket therefore describes how effectively energy can flow into it.
The differential electrical input works in the same manner. It only responds when there is a differential signal present, which is when there is a difference between the voltage on the positive and negative input terminals.
If the same voltage is present on each, as you might have from noise that gets coupled into the connecting cable, no input signal is realized across the input terminals. This is how differential inputs reject external noise.
The electrical circuit behaves in the same manner as the mechanical, exhibiting high common-mode impedance to noise signals common to both inputs.