Gain Setting via GAIN0 and GAIN1 Inputs
INPUT RESISTANCE
C
i
IN
Z
i
Z
f
Input
Signal
f =
1
2 Z C
p
i
i
(1)
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SLOS578 – MAY 2008
Several things can be done to relieve power-supply pumping. The lowest impact is to operate the two inputs out
of phase 180
°
and reverse the speaker connections. Because most audio is highly correlated, this causes the
supply pumping to be out of phase and not as severe. If this is not enough, the amount of bulk capacitance on
the supply must be increased. Also, improvement is realized by hooking other supplies to this node, thereby,
sinking some of the excess current. Power-supply pumping should be tested by operating the amplifier at low
frequencies and high output levels.
The gain of the TPA3124D2 is set by two input terminals, GAIN0 and GAIN1.
The gains listed in
are realized by changing the taps on the input resistors and feedback resistors inside
the amplifier. This causes the input impedance (Z
I
) to be dependent on the gain setting. The actual gain settings
are controlled by ratios of resistors, so the gain variation from part-to-part is small. However, the input impedance
from part-to-part at the same gain may shift by ±20% due to shifts in the actual resistance of the input resistors.
For design purposes, the input network (discussed in the next section) should be designed assuming an input
impedance of 8 k
Ω
, which is the absolute minimum input impedance of the TPA3124D2. At the higher gain
settings, the input impedance could increase as high as 72 k
Ω
.
Table 2. Gain Setting
INPUT IMPEDANCE
AMPLIFIER GAIN (dB),
GAIN1
GAIN0
(k
Ω
),
TYPICAL
TYPICAL
0
0
20
60
0
1
26
30
1
0
32
15
1
1
36
9
Changing the gain setting can vary the input resistance of the amplifier from its smallest value, 10 k
Ω
±20%, to
the largest value, 60 k
Ω
±20%. As a result, if a single capacitor is used in the input high-pass filter, the –3-dB
cutoff frequency may change when changing gain steps.
The –3-dB frequency can be calculated using
. Use the Z
I
values given in
Copyright © 2008, Texas Instruments Incorporated
13
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