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CiIN Zi
Zf
Input
Signal
f1
2ZiCi
(5)

INPUT CAPACITOR, C

I
fc1
2ZiCi
−3 dB
fc
(6)
Ci1
2Zifc
(7)
Power Supply Decoupling,C
S
TPA3008D2
SLOS435A – MAY 2004 – REVISED JULY 2004
The -3-dB frequency can be calculated using Equation 5. Use Table 1 for Z
i
values.
In the typical application, an input capacitor (C
i
) is required to allow the amplifier to bias the input signal to theproper dc level for optimum operation. In this case, C
i
and the input impedance of the amplifier (Z
i
) form ahigh-pass filter with the corner frequency determined in Equation 6 .
The value of C
i
is important, as it directly affects the bass (low-frequency) performance of the circuit. Considerthe example where Z
i
is 137 kΩand the specification calls for a flat bass response down to 20 Hz. Equation 6 isreconfigured as Equation 7 .
In this example, C
i
is 58 nF; so, one would likely choose a value of 0.1 µF as this value is commonly used. If thegain is known and is constant, use Z
i
from Table 1 to calculate C
i
. A further consideration for this capacitor is theleakage path from the input source through the input network (C
i
) and the feedback network to the load. Thisleakage current creates a dc offset voltage at the input to the amplifier that reduces useful headroom, especiallyin high gain applications. For this reason, a low-leakage tantalum or ceramic capacitor is the best choice. Whenpolarized capacitors are used, the positive side of the capacitor should face the amplifier input in mostapplications as the dc level there is held at 2.5 V, which is likely higher than the source dc level. Note that it isimportant to confirm the capacitor polarity in the application.
For the best pop performance, C
I
should be less than or equal to 1µF.
The TPA3008D2 is a high-performance CMOS audio amplifier that requires adequate power supply decouplingto ensure that the output total harmonic distortion (THD) is as low as possible. Power supply decoupling alsoprevents oscillations for long lead lengths between the amplifier and the speaker. The optimum decoupling isachieved by using two capacitors of different types that target different types of noise on the power supply leads.For higher frequency transients, spikes, or digital hash on the line, a good low equivalent-series-resistance (ESR)ceramic capacitor, typically 0.1 µF placed as close as possible to the device V
CC
lead works best. For filteringlower frequency noise signals, a larger aluminum electrolytic capacitor of 10 µF or greater placed near the audiopower amplifier is recommended. The 10-µF capacitor also serves as local storage capacitor for supplyingcurrent during large signal transients on the amplifier outputs.
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