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The 4 bit D to A
converter consists of
four resistors, whose
values are in the ratios
shown in the left hand
diagram above.
They can go high or
low, as shown in the
right diagram.

The voltage out is
determined by which of A
B C D are high and which
are low.
The resistors are
usually not "hard wired"
high or low as shown,
but taken high or low by
signal voltages.

The digital signals at A
B C D can be between
0000 and 1111, which
gives 16 steps, suitable
for a volume control
etc.
If finer resolution is
required, then 8 bits
can give 256 steps.
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