The "10x rule" for impedance bridging
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.