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LINEARITY, OFFSET, AND AGAIN ERROR USING SINGLE END SUPPLIES

DAC Code

Output

Voltage

0 V

Negative

Offset

TLV5633C

TLV5633I

SLAS190C – MARCH 1999 – REVISED SEPTEMBER 2006

Figure 13. TLV5633 Board Layout

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, because the most negative

supply rail is ground, the output cannot drive below ground and clamps the output at 0 V.

The output voltage remains at zero until the input code value produces a sufficient positive output voltage to

overcome the negative offset voltage, resulting in the transfer function shown in Figure 14.

Figure 14. Effect of Negative Offset (Single Supply)

The 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 the ground rail.

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 breakpoint in the transfer function. So the linearity is

measured between full scale code and the lowest code that produces a positive output voltage.

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