M41T00AUD
Audio section operation
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7.1 Gain
The programmable gain stage follows the band pass filter. It provides between –33 and
+12dB of gain, in 3dB steps (+/-1dB per step). The gain is selected by the GAIN bits, D3-D0
of register 08h, as listed in
Table 4
. A MUTE bit, D4 of the same register, allows the audio to
be cut off altogether.
At the first power up, GAIN will be initialized to its lowest value, 0, corresponding to a gain of
–33dB. Furthermore, MUTE will be set thus cutting off all audio.
On subsequent power ups, GAIN is unaffected, but the MUTE bit is always set to turn off the
audio at power up.
The final section is the output driver. It has a differential output capable of driving 300mW
into an 8
Ω
load.
The overall gain of the M41T00AUD is defined as the ratio of the AC output voltage, A
OUT
,
and the AC input voltage, S
IN
, as shown in
Figure 16
.
The 0.1uF input coupling capacitor
blocks any DC in the input signal.
Equation 1
Overall gain = A
OUT
/ S
IN
A
OUT
is measured between the output pins AOUT
+
and AOUT
–
.
A
OUT
= AOUT
+
-
AOUT
–
Each of the output levels is determined by the ratio of the feedback and input resistors along
with the GAIN value.
AOUT
+
= S
IN
x A
V
x R2/R1
AOUT
–
=
-
S
IN
x A
V
x R2/R1
where A
V
is the scalar gain as shown in
Table 7
.
Substituting these into Equation 1 above
yields:
A
OUT
= S
IN
x A
V
x R2/R1
-
(
-
S
IN
x A
V
x R2/R1) = 2 S
IN
x A
V
x R2/R1
With R1 = 2*R2, this reduces to A
OUT
= S
IN
x A
V
. Thus, when R1 = 2*R2, the gain levels in
Table 7
reflect overall gain of the circuit (at mid-band frequencies, about 1kHz with the
indicated 0.1uF capacitor). For GAIN set to B (0dB, A
V
= 1), the output voltage will be equal
to the input (
±
1dB).
7.1.1 Gain
tolerance
Two tolerance parameters apply to the gain levels. As shown in
Table 7
, upper and lower
limits are listed for four of the GAIN values (4, 5, Bh and Eh). For GAIN=Bh, the tolerance is
±1dB. This means the end-to-end gain of the part, with R1 = 2*R2, will be 0±1dB. For
GAIN=4, 5 and Eh, the tolerance is ±2dB. At each of these three settings, as shown in table
7, the gain will be within 2dB of the listed typical value. For GAIN =E, the end-to-end gain
will be b7 and +11 dB (9±2dB).
