WM8904ECSSLASHR datasheet(41/188 Pages) WOLFSON

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Pre-Production

WM8904

PP, Rev 3.3, September 2012

INPUT PGA COMMON MODE AMPLIFIER

In Differential Line Mode only, a Common Mode amplifier can be enabled as part of the input PGA

circuit. This feature provides approximately 20dB reduction in common mode noise on the differential

input, which can reduce problematic interference. Since the ADC has differential signal inputs, it has

an inherent immunity to common mode noise (see “Electrical Characteristics”) However, the presence

of Common Mode noise can limit the usable signal range of the ADC path; enabling the Common

Mode amplifier can solve this issue.

It should be noted that the Common Mode amplifier consumes additional power and can also add its

own noise to the input signal. For these reasons, it is recommended that the Common Mode Amplifier

is only enabled if there is a known source of Common Mode interference.

The Common Mode amplifier is controlled by the INL_CM_ENA and INR_CM_ENA fields as

described in Table 5. Although the Common Mode amplifier may be enabled regardless of the input

PGA mode, its function is only effective in the Differential Line Mode configuration.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R46 (2Eh)

Analogue Left

Input 1

INL_CM_ENA

Left Input PGA Common Mode

Rejection enable

0 = Disabled

1 = Enabled

(only available for L_MODE=01 –

Differential Line)

R47 (2Fh)

Analogue

Right Input 1

INR_CM_ENA

Right Input PGA Common Mode

Rejection enable

0 = Disabled

1 = Enabled

(only available for R_MODE=01 –

Differential Line)

Table 5 Common Mode Amplifier Enable

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