Can I adjust R4 and R5 to maintain a 12k input impedance?

[likephysics]

How do I reduce this resistor network.

The input current and the return current should be equal. But in this case, some current will also flow to ground. Can't get my head around this.

 

[berkeman]

How do I reduce this resistor network.

The input current and the return current should be equal. But in this case, some current will also flow to ground. Can't get my head around this.

No current flows "to ground" per se. Just connect the two ends of the resistors that have ground symbols with a wire -- that's the only other path for current. No current "leaves" the system the way it's drawn.

 

[likephysics]

So the two grounded resistors are in parallel with the 12k resistor?

 

[berkeman]

So the two grounded resistors are in parallel with the 12k resistor?

Yep!

 

[likephysics]

The 12K is the input impedance of a receiver IC. I got to increase R4,R5 to keep the impedance close to 12k.