APA2068
Application Descriptions (Cont.)
Consider the example where Ri is 10kW and the
Input Resistance, Ri (Cont.)
specification calls for a flat bass response down to
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
figure4.Theinputresistancewillaffectthelowfrequency
performance of audio signal. The minmum input
resistance is 10kW when gain setting is 20dB and the
resistance will ramp up when close loop gain below
20dB.The input resistance haswidevariation(+/-10%)
caused by process variation.
100Hz. Equation is reconfigured as follow :
1
Ci=
(5)
2px10kWxfC
Consider to input resistance variation, the Ci is 0.16mF
so one would likely choose a value in the range
Afurther 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 ceramiccapacitor isthe best choice. When polarized
capacitors are used, the positive side of the capacitor
should face the amplifier input in most applications as
theDC level thereisheld atVDD/2, whichislikelyhigher
that thesourceDC level. Pleasenotethat it isimportant
to confirm the capacitor polarity in the application.
Ri vs Gain(BTL)
Ri(k )
W
120
100
80
60
40
20
0
Effective BypassCapacitor, Cbypass
As other power amplifiers, proper supply bypassing is
critical for low noise performance and high power
supply rejection.
-40
-30
-20
-10
0
10
20
Gain(dB)
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 applicationsemploy a 5V regulator with 1.0mF
and a 0.1mF 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.
Figure 4: Input resistance vsGain setting
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 (10kW)
form a high-pass filter with the corner frequency
determined in the follow equation :
1
FC(highpass)=
(4)
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
2px10kWxCi
The value of Ci is important to consider as it directly
affects the low frequency performance of the circuit.
Copyright ã ANPEC Electronics Corp.
19
www.anpec.com.tw
Rev. A.2 - Jul., 2005
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
