Electronic – The “10x rule” for impedance bridging

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Quoting the Wikipedia impedance bridging article . . .

A connection is commonly said to be bridged if the load impedance is
at least ten times the source impedance.

Why the magical numer 10?

To give this question some context: If designing a preamp stage for audio equipment, am I better off designing an amp stage that has exactly 10x the nominal source impedance of the connecting device (perhaps using an inverting op-amp configuration with appropriate resistances), or is it just as good (if not preferable) to use a non-inverting op-amp configuration to give a gigantic load impedance?

Best Answer

Why the magical numer 10?

It's an order of magnitude in the decimal system.

Nothing magical happens there; it's a matter of diminishing returns.

If your input impedance is 10 times the source, you loose about 10% of the available source voltage in the source resistance.

If your input impedance is 100 times the source, you'd loose about 1%. 10 fold increase for 9% improvement

If your input impedance is 1000 times the source, you'd loose about 0.1%. 10 fold increase for 0.9% improvement.

There's no compelling reason to design for exactly 10X the nominal source impedance.

The art of engineering includes understanding approximations and when they are "good enough" or not.