WM8739 / WM8739L

Advanced Information

AI Rev 2.2 September 2001

13

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 are shown in the Figure 9.

AGND

AGND

R1

R2

C1

C2

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 at the input. R1 = 5K, R2= 5K, C1=47pF, C2=470nF (10V 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 WM8739 and the driving audio equipment. C2 together with the input

impedance of the WM8739 form a high pass filter.

ADC

The WM8739 uses a multi-bit

oversampled sigma-delta ADC. A single channel of the ADC is

illustrated in the Figure 10.

ANALOG

INTEGRATOR

FROM LINE

INPUT

MULTI

BITS

TO ADC DIGITAL FILTERS

Figure 10 Multi-Bit Oversampling Sigma Delta ADC Schematic

The use of multi-bit feedback and high oversampling rates reduces the effects of jitter and high

frequency noise.