Production Data

WM2623

WOLFSON MICROELECTRONICS LTD

PD Rev 1.0 October 2000

9

APPLICATIONS INFORMATION

LINEARITY, OFFSET, AND GAIN ERROR

Amplifiers operating from a single supply can have positive or negative voltage offsets. With a

positive offset, the output voltage changes on the first code transition. However, if the offset is

negative, the output voltage may not change with the first code, depending on the magnitude of the

offset voltage. This is because with the most negative supply rail being ground, any attempt to

drive the output amplifier below ground will clamp the output at 0 V. The output voltage then

remains at zero until the input code is sufficiently high to overcome the negative offset voltage,

resulting in the transfer function shown in Figure 7. <file negative_offset.emf, below>

DAC code

Negative

Offset

Output

Voltage

0 V

Figure 7 Effect of Negative Offset

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

the dotted line if the output buffer could drive below the ground rail.

DAC linearity is measured between zero-input code (all input bits at 0) and full-scale code (all

inputs at 1), disregarding offset and full scale errors. However, due to the breakpoint in the transfer

function, single supply operation does not allow for adjustment when the offset is negative. In such

cases, the linearity is therefore measured between full-scale and the lowest code that produces a

positive (non-zero) output voltage.

POWER SUPPLY DECOUPLING AND GROUNDING

Printed circuit boards with separate analogue and digital ground planes deliver the best system

performance. The two ground planes should be connected together at the low impedance power

supply source. Ground currents should be managed so as to minimise voltage drops across the

ground planes.

A 0.1

µF decoupling capacitor should be connected between the positive supply and ground pins of

the DAC, with short leads as close as possible to the device. Use of ferrite beads may further

isolate the system analogue supply from the digital supply.