WM5620L, WM5620
Applications Information
Linearity, offset, and gain error using
single end supplies
When an amplifier is operated from a single supply, the
voltage offset can still be either positive or negative. With
a positive offset, the output voltage changes on the first
code change. With a negative offset the output voltage may
not change with the first code depending on the magni-
tude of the offset voltage.
This negative offset error, not the linearity error, produces
this breakpoint. The transfer function would have followed
the dotted line if the output buffer could drive to a nega-
tive voltage.
For a DAC, linearity is measured between ZERO input code
( all inputs 0 ) and full scale code ( all inputs 1 ) after offset
and full scale are adjusted out or accounted for in some
way. However, single supply operation does not allow for
adjustment when the offset is negative due to the break-
point in the transfer function. So the linearity in the unipo-
lar mode is measured between full scale code and the
lowest code which produces a positive output voltage. The
code is calculated from the maximum specification for the
negative offset.
The output amplifier, with a negative voltage offset, attempts
to drive the output to a negative voltage. However, because
the most negative supply rail is GND, the output cannot
drive to a negative voltage.
So when the output offset voltage is negative, the output
voltage remains at ZERO volts until the input code value
produces a sufficient output voltage to overcome the
inherent negative offset voltage, resulting in the transfer
function shown below.
Effect of negative offset (single supply)
Wolfson Microelectronics
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