WM2631
Production Data
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 10MΩ 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
External (default)
1.024V
0
0
1
1
0
1
0
1
2.048V
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.
Output
Voltage
0 V
Negative
DAC code
Offset
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 0.1µF 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.
WOLFSON MICROELECTRONICS LTD
PD Rev 1.1 April 2001
12
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
