Wolfson Microelectronics

12

WM5620L, WM5620

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.

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)

Applications Information

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.