WM8737L Production Data
w
PD, Rev 4.4, January 2012
16
MICROPHONE PREAMPLIFER BYPASS AND BIAS CONTROL
When the Left or Right microphone preamplifier is disabled the user has two options for driving the
corresponding Left or Right PGA.
Default operation is to close a preamplifier bypass switch that connects the PGA input directly to the
L/RINPUT1/2/3 multiplexer output.
If the application has only a single left or right line level signal source and hence does not require the
multiplexer or microphone preamplifier, then better PGA gain accuracy and THD+N performance are
obtained by disabling the multiplexer and bypass switch and driving the PGA directly via the L/RACIN
pin. The multiplexer and switch are disabled by setting to zero the appropriate L/RBYPEN bit in
register R9. The L/RINPUT1/2/3 pads remain biased to VREF. These bits should be set to 1 if the
multiplexer is required (always required when the microphone preamplifier is enabled).
The user can also adjust the microphone preamplifier bias settings to optimise THD+N versus supply
current consumption for their application. Default is full bias for best THD+N performance, but the
user can reduce the bias to 75%, 50% or 25% of default by programming MBCTRL[1:0] in register R9.
REGISTER
ADDRESS
BIT LABEL DEFAULT DESCRIPTION
R9 (09h)
Microphone
Preamplifiers
Control
3
RBYPEN 1 Right channel bypass enable
0: Bypass switch is always open.
RINPUT1/2/3 high-impedance biased to
AVDD/2. RPGA input is RACIN pin.
1: Close bypass switch when right mic
preamp is disabled.
2
LBYPEN 1 Left channel bypass enable
0: Bypass switch is always open.
LINPUT1/2/3 high-impedance biased to
AVDD/2. LPGA input is LACIN pin.
1: Close bypass switch when left mic
preamp is disabled.
1:0
MBCTRL[1:0] 11 Bias control for left and right
microphone preamplifiers
00: bias 25%
01: bias 50%
10: bias 75%
11: nominal (100%) bias
Table 4 Microphone Preamplifier Control
MONO-MIXING
The WM8737L can operate as a stereo or mono device, or the two channels can be mixed to mono in
either the analogue domain (before the ADC) or in the digital domain (after the ADC). In all mono and
mono-mix modes unused circuitry can be switched off to save power (see “Power Management”
section). 3D stereo enhancement (See “3D Stereo Enhancement” section) is automatically disabled in
all mono and mono-mix modes.
In analogue mono-mix mode, the signals are mixed in the Left ADC and the Right ADC automatically
switches to dc measurement mode on pin RINPUT1. If dc measurement mode is not required then
the Right ADC can be powered down by setting bit 2 (ADCR) in the power management register R6.
Note that the high pass filter must be disabled if d.c. measurements are required.
In stereo and mono modes the Left/Right ADC data appear at the corresponding Left/Right Channel
outputs. In digital mono-mix mode the mixed data appears on both the Left and Right Channel
outputs. In analogue mono-mix mode the MONOUT bit controls whether the Right channel output
presents the data from the Right ADC (dc measurement) or a copy of the Left Channel (mixed)
output.
