Output impedance of the current output DAC
The output of a current-output R-2R ladder must be connected to a virtual ground, such as the inverting input of an opamp whose non-inverting input is grounded. Call the feedback resisor RF.

You can think of the circuit overall as "amplifying" the fixed VREF with a variable gain that is equal to

$$A = -\frac{R_F}{R_{OUT}}$$

and

$$V_{OUT} = -V_{REF}\frac{R_F}{R_{OUT}}$$

We know that

$$I_{OUT} = \frac{n}{2^N} \cdot \frac{V_{REF}}{R}$$

and

$$V_{OUT} = -R_F \cdot I_{OUT} = -R_F \cdot \frac{n}{2^N} \cdot \frac{V_{REF}}{R}$$

It's easy to set these two equations equal to each other and solve for ROUT:

$$-V_{REF}\frac{R_F}{R_{OUT}} = -R_F \cdot \frac{n}{2^N} \cdot \frac{V_{REF}}{R}$$

$$R_{OUT} = R \frac{2^N}{n}$$

When the input code is all-zeros, none of the R-2R legs are connected to the output node, so ROUT is infinite. When the input code is all-ones, ROUT is nearly equal to R.

It really doesn't matter, though, since as we said before, this output node needs to be connected to a virtual ground, so ROUT is in parallel with an impedance that is effectively zero. It's actual value has no effect on the rest of the the circuit.