adau1373 Analog Devices, Inc., adau1373 Datasheet - Page 39

• Manufacturer Part Number: adau1373
• Description: Low Power Codec With Speaker And Headphone Amplifier Adau1373
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
ANALOG INPUTS
The ADAU1373 provides four stereo (unbalanced)/mono
differential inputs. Each input gain and mode of operation can
be set independently, using the I
typical input block for the stereo and mono differential inputs in
the programmable gain amplifier (PGA) modes and boost modes.
Each input can receive either a microphone or a line level signal.
Each input can be set to either stereo single-ended mode or mono
differential mode. In addition, the gain for the input amplifiers
can be set to PGA mode or boost mode. In PGA mode, the gain
can be varied from −12 dB to +18 dB (in 1 dB steps); whereas in
boost mode, it can be varied using one of the following three steps:
0 dB, 9 dB, or 20 dB. In PGA mode, the input resistance varies
as per the gain, with a minimum of ~5.6 kΩ. In boost mode,
the input resistance is constant at ~20 kΩ.
Stereo Single-Ended Mode
In stereo single-ended mode, the AINxL/AINxP pins are used
as the left channel inputs, and the AINxR/AINxN pins are used
as the right channel inputs.
Figure 82. Typical Input Block for Stereo/Mono Differential Inputs in PGA Modes and Boost Modes
2
AINxN
AINxR
AINxP
C registers. Figure 82 shows the
AINxL
DIFFERENTIAL PGA MODE
STEREO PGA MODE
VCM
VCM
VCM
VCM
RIGHT MIX
LEFT MIX
RIGHT MIX
LEFT MIX
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AINxR
AINxN
AINxP
STEREO BOOST MODE
AINxL
DIFFERENTIAL BOOST MODE
Mono Differential Mode
In mono differential mode, the AINxL/AINxP pins are used as
positive inputs, and the AINxR/AINxN pins are used as negative
inputs for the differential amplifier.
The input amplifers use AVDD as the supply voltage. The
AINxP/AINxN input pins are internally biased to AVDD/2,
which is the common-mode voltage for the amplifiers. The
common-mode pin (CM, Ball B9) must be decoupled using the
10 μF electrolytic capacitor, as well as a 100 nF, X7R ceramic
capacitor to keep the reference clean for lower noise. In addition,
the input pins must be provided with an ac coupling capacitor to
the desired source.
The typical value of the coupling capacitor can be calculated
using the desired 3 dB roll-off frequency, as follows:
where R
Typically, a 2.2 μF capacitor is recommended. This sets the
lower frequency cutoff, at 20 Hz, at approximately −1.5 dB.
Frequency f (3 dB) = 1/(2 × π × R
VCM
VCM
VCM
VCM
IN
= 5.6 kΩ.
RIGHT MIX
RIGHT MIX
LEFT MIX
LEFT MIX
IN
× C
IN
)
ADAU1373
(1)