APA2068 Anpec Electronics Coropration, APA2068 Datasheet - Page 19

• Manufacturer Part Number: APA2068
• Description: Stereo 2.6W Audio Power Amplifier (with DC_Volume Control)
• Manufacturer: Anpec Electronics Coropration

APA2068
Input Resistance, Ri (Cont.)
BTL mode operation brings the factor of 2 in the gain
equation due to the inverting amplifier mirroring the
voltage swing across the load. For the varying gain
setting, APA2068 generates each input resistance on
figure 4. The input resistance will affect the low frequency
performance of audio signal. The minmum input
resistance is 10k when gain setting is 20dB and the
resistance will ramp up when close loop gain below
20dB. The input resistance has wide variation (+/-10%)
caused by process variation.
Input Capacitor, Ci
In the typical application an input capacitor, Ci, is
required to allow the amplifier to bias the input signal
to the proper DC level for optimum operation. In this
case, Ci and the minimum input impedance Ri (10k )
form a high-pass filter with the corner frequency
determined in the follow equation :
The value of Ci is important to consider as it directly
affects the low frequency performance of the circuit.
Copyright
Rev. A.2 - Jul., 2005
Ri(k )
Application Descriptions (Cont.)
120
100
80
60
40
20
0
-40
F
C
Figure 4: Input resistance vs Gain setting
(highpass)=
-30
ANPEC Electronics Corp.
2 x10k xCi
-20
1
-10
0
Ri vs Gain(BTL)
(4)
10
Gain(dB)
20
19
Consider the example where Ri is 10k
specification calls for a flat bass response down to
100Hz. Equation is reconfigured as follow :
Consider to input resistance variation, the Ci is 0.16 F
so one would likely choose a value in the range
A further consideration for this capacitor is the leakage
path from the input source through the input network
(Ri+Rf, Ci) to the load. This leakage current creates a
DC offset voltage at the input to the amplifier that
reduces useful headroom, especially in high gain
applications. For this reason a low-leakage tantalum
or ceramic capacitor is the best choice. When polarized
capacitors are used, the positive side of the capacitor
should face the amplifier input in most applications as
the DC level there is held at V
that the source DC level. Please note that it is important
to confirm the capacitor polarity in the application.
Effective Bypass Capacitor, Cbypass
As other power amplifiers, proper supply bypassing is
critical for low noise performance and high power
supply rejection.
The capacitors located on both the bypass and power
supply pins should be as close to the device as
possible. The effect of a larger bypass capacitor will
improve PSRR due to increased supply stability.
Typical applications employ a 5V regulator with 1.0 F
and a 0.1 F bypass capacitor as supply filtering. This
does not eliminate the need for bypassing the supply
nodes of the APA2068. The selection of bypass
capacitors, especially Cbypass, is thus dependent
upon desired PSRR requirements, click and pop
performance.
To avoid start-up pop noise occurred, the bypass
voltage should rise slower than the input bias voltage
and the relationship shown in equation (6) should be
Ci=
2 x10k xf
1
C
DD
/2, which is likely higher
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(5)
and the