TLC7226C, TLC7226I, TLC7226M

QUADRUPLE 8BIT DIGITALTOANALOG CONVERTERS

SLAS060F − JANUARY 1995 − REVISED APRIL 2009

10

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

linearity, offset, and gain error using single-ended power supplies

When an amplifier is operated from a single power 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 ground, the output cannot be driven 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 a transfer

function shown in Figure 8.

DAC Code

Output

Voltage

0 V

Negative

Offset

Figure 8. Effect of Negative Offset (Single Power Supply)

This negative offset error, not the linearity error, produces the breakpoint. The transfer function would have

followed the dotted line if the output buffer could be driven 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 power supply operation does

not allow for adjustment when the offset is negative due to the breakpoint 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.