Datasheet

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The following table lists the meaning of the bits within the control register:

LINEARITY, OFFSET, AND GAIN ERROR USING SINGLE END SUPPLIES

DAC Code

Output

Voltage

0 V

Negative

Offset

TLV5639C

TLV5639I

SLAS189C – MARCH 1999 – REVISED JANUARY 2004

APPLICATION INFORMATION (continued)

D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

X X X X X X X REF1 REF0 X PWR SPD

(1)

(1) Default values

SPD : Speed control bit 1 = fast mode 0 = slow mode

PWR : Power control bit 1 = power down 0 = normal operation

REF1 and REF0 determine the reference source and the reference voltage.

REFERENCE BITS

REF1 REF0 REFERENCE

0 0 External

0 1 1.024 V

1 0 2.048 V

1 1 External

If an external reference voltage is applied to the REF pin, external reference must be selected.

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 12 .

Figure 12. 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 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.