WM8746
Production Data
It is important that the supply pins are connected correctly. If AVDD1 and AVDD2 pins had both been
connected to the 10Ω resistor, the performance would be worse. The value of 10Ω is too high and will
cause an increase in THD. This is because currents drawn by AVDD1 will affect the reference
voltage on AVDD2. A full-scale output FFT plot will show increased harmonics, because the output
current drawn modulates the reference voltage.
4.7µF ceramic capacitors are becoming available in 0805 package with Y5V dielectric. Whilst they do
not have quite as good performance as 10µF, it is possible to use them instead. Their capacitance
goes down significantly with supply voltage, but at 3.3V (VDD) and 1.65V (CAP pin), the drop is not
too great, if a 10V-rated part is used. As they are already in a low-inductance package, the 0.1µF is
no longer necessary. See Figure 26 for an example.
Figure 26 Decoupling Example 2
RECOMMENDED ANALOGUE LOW-PASS FILTER (OPTIONAL)
4.7kΩ
4.7kΩ
+VS
_
51
Ω
10u
F
1.8kΩ
7.5KΩ
+
+
-VS
1.0nF
680pF
10k
Ω
Figure 27 Recommended Low Pass Filter (Optional)
March 2006, PD Rev 4.0
30
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
