I have managed to confuse myself when it comes to calculating the voltage drop over a series resistor connected to a potential divider (See below):
The op amp non-inverting input is connected to a 20K resistor which is then connected to the output of a voltage divider (Top resistor is 56K, bottom is 7K). The input voltage to the potential divider is 12V.
I would like to calculate the voltage "seen" by the op amp (set as a voltage buffer) on its non-inverting input (in this case pin 3).
The input resistance for an ideal op amp should be infinite, however in practise there is a very minute input leakage current. This gives me the following questions:
1) Is the resistance "seen" on the potential divider output branch, equal to the sum of the op amp input resistance and 20k resistor? Or would it be equal to the equivalent parallel resistance of 7k||20k+op amp input resistance?
2) As the input resistance of the comparator is very high, (and I don't have specific values for it) could I somehow calculate the potential divider output voltage drops over the 20k resistor and op amp without calculating current draw?
3) Related to the previous questions, can I "split" the problem into two parts? i.e Calculate the potential divider output voltage and then calculate the voltage drop over the 20k resistor and op amp?
4) Would an equivalent circuit without the op amp (i.e 20k resistor is left floating on one side) help me calculate the voltage drop over the 20k resistor?
Thank you for your help- I've been driving myself mad for days with this problem...