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Chapter 21. Frequency-independant voltage devider


In one of the first lessons we have already learned about voltage deviders. These voltage deviders consisted of resistors only, and were therefore frequency-independant. However, when we want to connect a voltage devider to some measuring equipment, we'll have to take its input capacity into account.

In this chapter the 'measuring equipment' is an oscilloscope, which input impedance can be seen as a 1MΩ resistor parallel connected to a 30pF capacitor; this is usually written as 1M//30pF. At 159Hz the impedance of the 30pF capacitor is also 1MΩ. So the total impedance at 159Hz is just 500kΩ. The input-impedance is frequency-dependant. A 9MΩ resistor will not suffice to devide the voltage by 10. This wordt for DC voltages, but at 159Hz the voltage will not be devided by 10, but by (9M+500k)/500k = 19!

We will first create an HF probe that doesn't have this problem. Next, we will discover how this problem is solved in the attenuator of an oscilloscope.