# How to obtain both values from a voltage attenuator?

Design a resistive voltage attenuator whose input resistance be 1 MΩ (R1,R2) and able to divide by 5 the amplitude of a DC voltage when the output of the attenuator is connected to a circuit with 1 MΩ input resistance ($$\R_i\$$.)

The result is:

• R1=800kΩ
• R2=250kΩ

My question is how do I get those resistor values out a voltage attenuation equation?

• Welcome to EE.SE! Is this homework? Oct 9 '20 at 11:30
• no, is just an example I have come across, and after doing my maths my results are totally different. I was wondering if I was doing something wrong, that's all... Oct 9 '20 at 12:31
• You need to know how to work out the value of parallel resistors. Can you do that ? Oct 11 '20 at 0:26

So the total resistance of the divider R1 + R2 ||Ri = 1M$$\\Omega\$$, thus R2 || 1M$$\\Omega\$$ = 200K$$\\Omega\$$ for 0.2 division, so
R1 = 1M$$\\Omega\$$ - 200K$$\\Omega\$$ = 800K$$\\Omega\$$.
R2||1M$$\\Omega\$$ = 200K$$\\Omega\$$ so
1/R2 = (1/200K$$\\Omega\$$) - (1/1M$$\\Omega\$$) => R2 = 250K$$\\Omega\$$