WM8734 datasheet(14/42 Pages) ETC

14 / 42 page

WM8734

Advanced Information

AI Rev 2.2 November 2001

The line inputs are biased internally through the operational amplifier to VMID. Whenever the line

inputs are muted or the device placed into standby mode, the line inputs are kept biased to VMID

using special anti-thump circuitry. This reduces any audible clicks that may otherwise be heard when

re-activating the inputs.

The external components required to complete the line input application is shown in the Figure 9.

AGND

LINEIN

Figure 9 Line Input Application Drawing

For interfacing to a typical CD system, it is recommended that the input is scaled to ensure that there

is no clipping of the signal. R1 = 5K, R2 = 5K, C1 = 47pF, C2 = 470nF (10V npo ceramic type).

R1 and R2 form a resistive divider to attenuate the 2 Vrms output from a CD player to a 1 Vrms level,

so avoiding overloading the inputs. R2 also provides a discharge path for C2, thus preventing the

input to C2 charging to an excessive voltage which may otherwise damage any equipment connected

that is not suitably protected against high voltages. C1 forms an RF low pass filter for increasing the

rejection of RF interference picked up on any cables. C2 forms a DC blocking capacitor to remove

the DC path between the WM8734 and the driving audio equipment. C2 together with the input

impedance of the WM8734 form a high pass filter.

ADC

The WM8734 uses a multi-bit oversampled sigma-delta ADC. A single channel of the ADC is

illustrated in the Figure 10.

FROM LINE INPUT

ANALOG

INTEGRATOR

MULTI

BITS

TO ADC DIGITAL FILTERS

Figure 10 Multi-Bit Oversampling Sigma Delta ADC Schematic

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