Compensated Attenuator

The purpose of a compensated attenuator is to provide a uniform attenuation factor for all frequencies. Attenuators are found in many systems, particularly test instruments. They allow input quantities to be reduced to a level compatible with the range acceptable to a system. This must be done with a flat response across the frequency range. To achieve this the scaling resistance is shunted with a capacitor to compensate for the capacitance present at the input of the system. The example in this circuit is common to oscilloscopes using a x10 probe. Rin and Cin represent the resistance and capacitance seen at the input terminal of the instrument. Ra and Ca represent the resistance and capacitance of the attenuator (x10 probe in the case of the oscilloscope). To provide a flat attenuation factor of a, Ra should be (a - a) times greater than Rin and Ca should be (a - 1) times smaller than Cin. For a x10 probe (a - 1) = 10 - 1 = 9. Note: It appears on the graph that only the magnitude of the output voltage at node 2 is flat. It looks like attenuation is accompanied by a large phase shift which peaks at 6.3096 kHz but the scale of phase is in µ° with peak of only 25.724 µ°.