WM8521

Production Data

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PD Rev 4.1 August 2006

18

RECOMMENDED ANALOGUE LOW PASS FILTER

Ω

Ω

Ω

Ω

Figure 19 Recommended 1st Order Low Pass Filter

Note: Capacitors should be COG dielectric.

An external single pole RC filter is recommended (see Figure 19) if the device is driving a wideband

amplifier. However the WM8521 does contain an internal low pass filter which should be adequate in

most applications.

PCB LAYOUT RECOMMENDATIONS

Care should be taken in the layout of the PCB that the WM8521 is to be mounted to. The following

notes will help in this respect:

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.

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 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.

Separate analogue and digital track routing from each other. The device is split into analogue

(pins 5 – 10) and digital (pins 1 – 4 & pins 11 – 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.

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 WM8521 that demonstrates the above techniques and the

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