WM5628L, WM5628

Wolfson Microelectronics

13

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

Effect of negative offset (single supply)

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.