TLC7225C, TLC7225I

QUADRUPLE 8-BIT DIGITAL-TO-ANALOG CONVERTERS

SLAS109B – OCTOBER 1996 – REVISED FEBRUARY 2001

19

POST OFFICE BOX 655303

APPLICATION INFORMATION

linearity, offset, and gain error using single-ended 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 attempts to drive the output to a negative voltage. However, since the most negative supply

rail is ground, the output cannot drive below ground.

So with this output offset voltage, the output voltage remains at zero until the input-code value produces a

sufficient output voltage to overcome the inherent offset voltage, resulting in a transfer function shown in

Figure 18.

DAC Code

Output

Voltage

0 V

Negative

Offset

Figure 18. Effect of Negative Offset (Single Supply)

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

dotted line if the output buffer could drive below ground.

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 is 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 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 zero offset error.