WM2629
APPLICATIONS INFORMATION
LINEARITY, OFFSET, AND GAIN ERROR
Production Data
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
0V
Negative
Offset
DAC code
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 1.0 April 2001
12
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
