Pre-Production
WM8904
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.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R46 (2Eh)
INL_CM_ENA
Left Input PGA Common Mode
Rejection enable
6
1
Analogue Left
Input 1
0 = Disabled
1 = Enabled
(only available for L_MODE=01 –
Differential Line)
R47 (2Fh)
INR_CM_ENA
Right Input PGA Common Mode
Rejection enable
6
1
Analogue
Right Input 1
0 = Disabled
1 = Enabled
(only available for R_MODE=01 –
Differential Line)
Table 5 Common Mode Amplifier Enable
PP, Rev 3.3, September 2012
41
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
