Datasheet

ADAU1772 Data Sheet
Rev. B | Page 32 of 116
INPUT SIGNAL PATHS
There are four input paths, from either an ADC or a digital
microphone, that can be routed to the core. The input sources
(ADC or digital microphone) must be configured in pairs (for
example, 0 and 1, 2 and 3), but each channel can be routed
individually. The core inputs can also be sourced from a stereo
input ASRC.
ANALOG INPUTS
The ADAU1772 can accept both line level and microphone inputs.
Each of the four analog input channels can be configured in a
single-ended mode or a single-ended with PGA mode. There
are also inputs for up to four digital microphones. The analog
inputs are biased at AVDD/2. Unused input pins should be
connected to the CM pin or ac-coupled to ground.
Signal Polarity
Signals routed through the PGAs are inverted. As a result, signals
input through the PGA are output from the ADCs with a polarity
that is opposite that of the input. Single-ended inputs are not
inverted. The ADCs are noninverting.
Input Impedance
The input impedance of the analog inputs varies with the gain of
the PGA. This impedance ranges from 0.68 kΩ at the 35.25 dB
gain setting to 32.0 kΩ at the −12 dB setting. The input impedance
on each pin can be calculated as follows:
k
Ω
1
10
40
)
20/(
+
=
Gain
IN
R
where Gain is set by PGA_GAINx.
The optional 10 dB PGA boost set in PGA_x_BOOST does not
affect the input impedance. This is an alternative way of increasing
gain without decreasing input impedance; however, it causes some
degradation in performance.
Analog Microphone Inputs
For microphone signals, the ADAU1772 analog inputs can be
configured in single-ended with PGA mode.
The PGA settings are controlled in Register 0x0023 to Register
0x0026. The PGA is enabled by setting the PGA_ENx bits.
Connect the AINxREF pins to the CM pin and connect the
microphone signal to the inverting input of the PGAs (AINx),
as shown in Figure 70.
Figure 70. Single-Ended Microphone Configuration
Analog Line Inputs
Line level signals can be input on the AINx pins of the analog
inputs. Figure 71 shows a single-ended line input using the
AINx pins. The AINxREF pins should be tied to CM. When using
single-ended line input, the PGA should be disabled using the
PGA_ENx bits, and the corresponding PGA pop suppression
bit should be disabled using the POP_SUPPRESS register
(Address 0x0029).
Figure 71. Single-Ended Line Inputs
Precharging Input Capacitors
Precharge amplifiers are enabled by default to quickly charge
large series capacitors on the inputs and outputs. Precharging
these capacitors helps to prevent pops in the audio signal. The
precharge circuits are powered up by default on startup and can be
disabled in the POP_SUPPRESS register. The precharge amplifiers
are automatically disabled when the PGA or headphone amplifiers
are enabled. For unused PGAs and headphone outputs, these
precharge amplifiers should be disabled using the POP_SUPPRESS
register. The precharging time is dependent on the input/output
series capacitors. The impedance looking into the pin is 500 Ω
in this mode. However, at startup, the impedance looking into
the pin is dominated by the time constant of the CM pin because
the precharge amplifiers reference the CM voltage.
Microphone Bias
The ADAU1772 includes two microphone bias outputs: MICBIAS0
and MICBIAS1. These pins provide a voltage reference for electret
analog microphones. The MICBIASx pins can also be used to
cleanly supply voltage to digital or analog MEMS microphones
with separate power supply pins. The MICBIASx voltage is set
in the microphone bias control register (Address 0x002D). Using
this register, either the MICBIAS0 or MICBIAS1 output can be
enabled and disabled. The gain options provide two possible
voltages: 0.65 × AVDD or 0.90 × AVDD.
Many applications require enabling only one of the two bias
outputs. The two bias outputs should both be enabled when many
microphones are used in the system or when the positioning of
the microphones on the PCB does not allow one pin to bias all
microphones.
MICROPHONE
PGA
–12dB TO
+35.25dB
ADAU1772
AINx
AINxREF
MICBIASx
CM
2kΩ
10804-063
ADAU1772
LINE INPUT 0 AIN0
LINE INPUT 1 AIN1
LINE INPUT 2 AIN2
LINE INPUT 3 AIN3
10804-064