Audio section operation M41T00AUD
30/42 Doc ID 13480 Rev 5
7.1 Gain
The programmable gain stage follows the band pass filter. It provides between –33 and
+12 dB of gain, in 3 dB steps (+/-1 dB per step). The gain is selected by the GAIN bits, D3-
D0 of register 08h, as listed in Tab l e 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
–33 dB. 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 0.1 uF 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 Ta ble 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 0.1 uF capacitor). For GAIN set to B (0 dB, A
V
= 1), the output voltage will be
equal to the input (
±1 dB).
7.1.1 Gain tolerance
Two tolerance parameters apply to the gain levels. As shown in Tab l e 7 , upper and lower
limits are listed for four of the GAIN values (4, 5, Bh and Eh). For GAIN=Bh, the tolerance is
±1 dB. This means the end-to-end gain of the part, with R1 = 2*R2, will be 0±1 dB. For
GAIN = 4, 5 and Eh, the tolerance is ±2 dB. At each of these three settings, as shown in
table 7, the gain will be within 2 dB of the listed typical value. For GAIN =E, the end-to-end
gain will be between +7 and +11 dB (9±2 dB).
