Simple circuit - equivalent impedance - amateur question

Thread Starter


Joined Apr 21, 2022
Hi, i'm currently teaching myself electronics; In doing so i've encountered a circuit which my current grasp of theory is failing to understand. The circuit is derived from a 3bit R2R ladder, bits 1&2 are set high. Here's an ascii pictorial version of the circuit:

Vcc is 5V
R1 is 3.3K, R2 is 6.8K


My usual approach is to reduce the parallel portions of the circuit and simplify, so that I can calculate the total impedance and then the current, then the voltage drops/voltages at each nodes.

The above circuit slightly simplified:
And again:

So from this point my understanding of theory fails me. I can only arrive at the correct (measured at 7.4K) value by totally ignoring R1, calculating:

(R2 + R2) in parallel with (R2 + R3), i.e.
R4 in parallel with R5, i.e.
13.6K in parallel with 16.9K = 7.54k!

This is the closest I can calculate to the measured value, however of course R1 is missing. I'd greatly appreciate any help and theory corrections.