SI4720-B20-GMR Datasheet, PDF (31/48 Pages) Silicon Laboratories – BROADCAST FM RADIO TRANSCEIVER FOR PORTABLE APPLICATIONS

Si4720/21-B20
(IFALL = 1)
INVERTED
DCLK
(IFALL = 0)
DCLK
Left-Justified
(IMODE = 0110)
DFS
DIN/DOUT
LEFT CHANNEL
RIGHT CHANNEL
1
2
3
n-2
n-1
n
1
2
3
MSB
LSB
MSB
Figure 22. Left-Justified Digital Audio Format
n-2
n-1
n
LSB
(IFALL = 0)
DCLK
DFS
(IMODE = 1100)
DIN/DOUT
(MSB at 1^st rising edge)
DIN/DOUT
(IMODE = 1000) (MSB at 2^nd rising edge)
1
MSB
1 DCLK
LEFT CHANNEL
RIGHT CHANNEL
2
3
n-2
n-1
n
1
2
3
n-2
n-1
LEFT CHANNEL
LSB
MSB
RIGHT CHANNEL
1
2
3
n-2
n-1
n
1
2
3
n-2
MSB
LSB
MSB
Figure 23. DSP Digital Audio Format
n
LSB
n-1
n
LSB
5.17. Line Input
The Si4720/21 provides left and right channel line inputs
(LIN and RIN). The inputs are high-impedance and low-
capacitance, suited to receiving line level signals from
external audio baseband processors. Both line inputs
are low-noise inputs with programmable attenuation.
Passive and active anti-aliasing filters are incorporated
to prevent high frequencies from aliasing into the audio
band and degrading performance.
To ensure optimal audio performance, the Si4720/21
has a TX_LINE_INPUT_LEVEL property that allows the
user to specify the peak amplitude of the analog input
(LILEVEL[9:0]) required to reach the maximum
deviation level programmed in the audio deviation
property, TX_AUDIO_DEVIATION. A corresponding line
input attenuation code, LIATTEN[1:0], is also selected
by the expected peak amplitude level. Table 18 shows
the line attenuation codes.
Table 18. Line Attenuation Codes
LIATTEN[1:0]
00
01
10
11
Peak Input
Voltage [mV]
190
301
416
636
RIN/LIN Input
Resistance [k]
396
100
74
60
The line attenuation code is chosen by picking the
lowest Peak Input Voltage in Table 18 that is just above
the expected peak input voltage coming from the audio
baseband processor. For example, if the expected peak
input voltage from the audio baseband processor is
400 mV, the user chooses LIATTEN[1:0] = 10 since the
Peak Input Voltage of 416 mV associated with
LIATTEN[1:0] = 10 is just greater than the expected
peak input voltage of 400 mV. The user also enters
400 mV into the LILEVEL[9:0] to associate this input
level to the maximum frequency deviation level
programmed into the audio deviation property. Note that
selecting a particular value of LIATTEN[1:0] changes
the input resistance of the LIN and RIN pins. This
feature is used for cases where the expected peak input
level exceeds the maximum input level of the LIN and
RIN pins.
The maximum analog input level is 636 mVpK. If the
analog input level from the audio baseband processor
exceeds this voltage, series resistors must be inserted
in front of the LIN and RIN pins to attenuate the voltage
such that it is within the allowable operating range. For
example, if the audio baseband's expected peak
amplitude is 900 mV and the V_IO supply voltage is 1.8 V,
the designer can use 30 k series resistors in front of
the LIN and RIN pins and select LIATTEN[1:0] = 11. The
resulting expected peak input voltage at the LIN/RIN
pins is 600 mV, since this is just a voltage divider
between the LIN/RIN input resistance (see Table 18,
60 k for this example) and the external resistor. Note
that the Peak Input Voltage corresponding to the chosen
Rev. 1.0
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