WM8726

Production Data

w

PD, Rev 4.8, January 2012

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

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 WM8726 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 WM8726 that demonstrates the above techniques and the

excellent performance achievable from the device. This can be ordered or the User manual