WM2331

Production Data

WOLFSON MICROELECTRONICS LTD

PD Rev 1.4 April 2001

24

REFERENCE DECOUPLING

VREF PIN

When the on-chip reference generator is enabled, the VREF pin should be decoupled to the circuit

board’s analogue ground plane close to the WM2331 AGND pin via a 1

µF tantalum capacitor and a

0.1

µF ceramic capacitor.

REFTF AND REFBF PINS

In any mode of operation, the REFTF and REFBF pins should be decoupled as shown in Figure 28

below. Use short board traces between the WM2331 and the capacitors to minimise parasitic

inductance.

WM2331

REFTF

REFBF

0.1µF

0.1µF

0.1µF

10µF

Figure 28 Recommended Decoupling for the ADC Reference Pins REFTF and REFBF

SUPPLY DECOUPLING

The analogue (AVDD, AGND) and digital (DVDD, DGND) power supplies to the WM2331 should be

separately decoupled for best performance. Each supply needs at least a 10

µF electrolytic or

tantalum capacitor (as a charge reservoir) and a 100nF ceramic type capacitor placed as close as

possible to the respective pins (to suppress spikes and supply noise).

DIGITAL OUTPUT LOADING AND CIRCUIT BOARD LAYOUT

The WM2331 outputs are capable of driving rail-to-rail with up to 20pF of load per pin at 30MHz clock

and 3V digital supply. Minimising the load on the outputs will improve WM2331 signal-to-noise

performance by reducing the switching noise coupling from the WM2331 output buffers to the internal

analogue circuits. The output load capacitance can be minimised by buffering the WM2331 digital

outputs with a low input capacitance buffer placed as close to the output pins as physically possible,

and by using the shortest possible tracks between the WM2331 and this buffer.

Noise levels at the output buffers, which may affect the analogue circuits within WM2331, increase

with the digital supply voltage. Where possible, consider using the lowest DVDD that the application

can tolerate.

Use good layout practices when designing the application PCB to ensure that any off-chip return

currents from the WM2331 digital outputs (and any other digital circuits on the PCB) do not return via

the supplies to any sensitive analogue circuits. The WM2331 should be soldered directly to the PCB

for best performance. Socketing the device will degrade performance by adding parasitic socket

inductance and capacitance to all pins.

USER TIPS FOR OBTAINING BEST PERFORMANCE FROM THE WM2331

•

Choose differential input mode for best distortion performance.

•

Choose a 2V ADC input span for best noise performance.

•

Choose a 1V ADC input span for best distortion performance.

•

Drive the clock input CLK from a low-jitter, fast logic stage, with a well-decoupled

power supply and short PCB traces.