WM2632

Production Data

WOLFSON MICROELECTRONICS LTD

PD Rev 1.1 April 2001

12

PROGRAMMABLE INTERNAL REFERENCE

The reference can be sourced internally or externally under software control. If an external reference

voltage is applied to the REF pin, the device must be configured to accept this.

If an external reference is selected, the reference voltage input is buffered which makes the DAC

input resistance independent of code. The REF pin has an input resistance of 10M

Ω and an input

capacitance of typically 55pF. The reference voltage determines the DAC full-scale output.

If an internal reference is selected, a voltage of 1.024V or 2.048 is available. The internal reference

can source up to 1mA and can therefore be used as an external system reference.

REF1

REF0

REFERENCCE

0

0

External (default)

0

1

1.024V

1

0

2.048V

1

1

External

Table 9 Programmable Internal Reference

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 10.

DAC code

Negative

Offset

Output

Voltage

0 V

Figure 10 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.