TLV5625

2.7-V TO 5.5-V LOW-POWER DUAL 8-BIT DIGITAL-TO-ANALOG

CONVERTER WITH POWER DOWN

SLAS233A – JULY 1999 – REVISED MARCH 2000

12

POST OFFICE BOX 655303

APPLICATION INFORMATION

examples of operation (continued)

Both outputs are updated on the rising clock edge after D0 from the DAC A data word is sampled.

D15

D14

D13

D12

D11

D10

D9

D8

D7

D6

D5

D4

D3

D2

D1

D0

X

X

1

X

X

X

X

X

X

X

X

X

X

X

X

X

X = Don’t care

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, because the most negative

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

The output voltage then 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 13.

DAC Code

Output

Voltage

0 V

Negative

Offset

Figure 13. 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.

power-supply bypassing and ground management

Printed-circuit boards that use separate analog and digital ground planes offer the best system performance.

Wire-wrap boards do not perform well and should not be used. The two ground planes should be connected

together at the low-impedance power-supply source. The best ground connection may be achieved by

connecting the DAC AGND terminal to the system analog ground plane, making sure that analog ground

currents are well managed and there are negligible voltage drops across the ground plane.

A 0.1-

as close as possible to the device. Use of ferrite beads may further isolate the system analog supply from the

digital power supply.

Figure 14 shows the ground plane layout and bypassing technique.