EVAL-ADAU1401EB AD [Analog Devices], EVAL-ADAU1401EB Datasheet - Page 19

• Manufacturer Part Number: EVAL-ADAU1401EB
• Description: SigmaDSP 28-/56-Bit Audio Processor with Two ADCs and Four DACs
• Manufacturer: AD [Analog Devices]

AUDIO ADCs
The ADAU1401 has two Σ-Δ ADCs. The signal-to-noise ratio
(SNR) of the ADCs is 100 dB, and the THD + N is −83 dB.
The stereo audio ADCs are current input; therefore, a voltage-
to-current resistor is required on the inputs. This means that
the voltage level of the input signals to the system can be set to
any level; only the input resistors need to be scaled to provide
the proper full-scale current input. The ADC0 and ADC1 input
pins, as well as ADC_RES, have an internal 2 kΩ resistor for
ESD protection. The voltage seen directly on the ADC input
pins is the 1.5 V common mode.
The external resistor connected to ADC_RES sets the full-scale
current input of the ADCs. The full range of the ADC inputs is
100 μA
total, because it is in series with the internal 2 kΩ). The only
reason to change the ADC_RES resistor is if a sampling rate
other than 48 kHz is used.
The voltage-to-current resistors connected to ADC0/ADC1 set
the full-scale voltage input of the ADCs. With a full-scale current
input of 100 μA
(in series with the 2 kΩ internal resistor) results in an input using
the full range of the ADC. The matching of these resistors to the
ADC_RES resistor is important to the operation of the ADCs.
For these three resistors, a 1% tolerance is recommended.
Either the ADC0 and/or ADC1 input pins can be left
unconnected if that channel of the ADC is unused.
These calculations of resistor values assume a 48 kHz sample
rate. The recommended input and current setting resistors
scale linearly with the sample rate because the ADCs have a
switched-capacitor input. The total value (2 kΩ internal plus
external resistor) of the ADC_RES resistor with sample rate
f
S_NEW
R
can be calculated as follows:
total
rms
with an external 18 kΩ resistor on ADC_RES (20 kΩ
=
20
rms,
kΩ
a 2.0 V
×
f
48
S
_
,
000
rms
NEW
signal with an external 18 kΩ resistor
Rev. 0 | Page 19 of 52
The values of the resistors (internal plus external) in series with
the ADC0 and ADC1 pins can be calculated as follows:
Table 14 lists the external and total resistor values for common
signal input levels at a 48 kHz sampling rate. A full-scale rms
input voltage of 0.9 V is shown in the table because a full-scale
signal at this input level is equal to a full-scale output on the DACs.
Table 14. ADC Input Resistor Values
Full-Scale
RMS Input
Voltage (V)
0.9
1.0
2.0
Figure 17 shows a typical configuration of the ADC inputs for
a 2.0 V
used to ac-couple the signals so that the inputs are biased at 1.5 V.
R
rms
Input
input signal for a f
Total
Figure 17. Audio ADC Input Configuration
ADC_RES
Value (kΩ)
18
18
18
=
(
RMS
47µF
47µF
Input
S
of 48 kHz. The 47 μF capacitors are
Voltage
ADC0/ADC1
Resistor
Value (kΩ)
7
8
18
18kΩ
18kΩ
18kΩ
ADAU1401
)
×
ADC0
ADC_RES
ADC1
10
kΩ
9
Total ADC0/ADC1
Input Resistance
(External +
Internal) (kΩ)
10
20
×
ADAU1401
48
f
S
_
,
000
NEW