WM8761 Production Data
w
PD, Rev 4.6, October 2011
20
RECOMMENDED ANALOGUE LOW PASS FILTER
+
_
+
+VS
-VS
10uF
51
7.5K
680pF
1.8k
47k
4.7k
4.7k
1.0nF
Figure 23 Recommended 2
nd
Order Low Pass Filter
An external low pass filter is recommended (see Figure 20) if the device is driving a wideband
amplifier. In some applications, a passive RC filter may be adequate.
PCB LAYOUT RECOMMENDATIONS
Care should be taken in the layout of the PCB that the WM8761 is to be mounted to. The
following notes will help in this respect:
1. The VDD supply to the device should be as noise free as possible. This can be
accomplished to a large degree with a 10uF bulk capacitor placed locally to the device and a
0.1uF high frequency decoupling capacitor placed as close to the VDD pin as possible. It is
best to place the 0.1uF capacitor directly between the VDD and GND pins of the device on
the same layer to minimize track inductance and thus improve device decoupling
effectiveness.
2. The CAP pin should be as noise free as possible. This pin provides the decoupling for
the on chip reference circuits and thus any noise present on this pin will be directly coupled
to the device outputs. In a similar manner to the VDD decoupling described in 1. above, this
pin should be decoupled with a 10uF bulk capacitor local to the device and a 0.1uF capacitor
as close to the CAP pin as possible.
3. Separate analogue and digital track routing from each other. The device is split into
analogue (pins 5 – 9) and digital (pins 1 – 4 & pins 10 – 14) sections that allow the routing of
these signals to be easily separated. By physically separating analogue and digital signals,
crosstalk from the PCB can be minimized.
4. Use an unbroken solid GND plane. To achieve best performance from the device, it is
advisable to have either a GND plane layer on a multilayer PCB or to dedicate one side of a
2 layer PCB to be a GND plane. For double sided implementations it is best to route as
many signals as possible on the device mounted side of the board, with the opposite side
acting as a GND plane. The use of a GND plane greatly reduces any electrical emissions
from the PCB and minimizes crosstalk between signals.
An evaluation board is available for the WM8761 that demonstrates the above techniques and the
excellent performance achievable from the device. This can be ordered or the User manual
downloaded from the Wolfson web site at www.wolfsonmicro.com
