WM5628L, WM5628
Functional Description (Continued)
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 magnitude of
the offset voltage.
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
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 negative
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 unipolar
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.
Effect of negative offset (single supply)
Wolfson Microelectronics
13
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
