Diodes PAM8404 - Manual (Page 12)

PAM8404
Document number: DSxxxxx Rev. 1 - 1
12 of 18
www.diodes.com
November 2012
© Diodes Incorporated
PAM8404
A Product Line of
Diodes Incorporated
Application Information
Test Setup for Performance Testing
Notes: 1. The AP AUX-0025 low pass filter is necessary for class-D amplifier measurement with AP analyzer.
2. Two 22μH inductors are used in series with load resistor to emulate the small speaker for efficiency measurement.
Gain Settin
The gain of PAM8404 can be selected as 6,12,18 or 24 dB utilizing the G0 and G1 gain setting pins. The gains showed in the following table are
realized by changing the input resistors inside the amplifier. The input impedance changes with the gain setting.
Table 1. Gain Setting
G1 G0
Gain
(V/V) Gain
(dB)
Input
Impedance
(kΩ)
0 0 2 6 28.1
0 1 4 12 17.3
1 0 8 18 9.8
1 1 16 24 5.2
For optimal performance the gain should be set to 2x (R
I
= 150kΩ). Lower gain allows the PAM8404 to operate at its best, and keeps a high
voltage at the input making the inputs less susceptible to noise. In addition to these features, lower value of Gain minimizes pop noise.
Input Capacitors (C
I
)
In the typical application, an input capacitor, C
I
, is required to allow the amplifier to bias the input signal to the proper DC level for optimum
operation. In this case, C
I
and the input impedance R
I
form a high-pass filter with the corner frequency determined by the follow equation:
C R 2
1
f
I I
C 

It is important to consider the value of C
I
as it directly affects the low frequency performance of the circuit. When R
i
i s 28.1kΩ and the
specification calls for a flat bass response are down to 200Hz, the equation is reconfigured as follows:
f R 2
1
C
C I
I 

When input resistance variation is considered, the C
I
is 28nF, so one would likely choose a value of 33nF. A further consideration for this
capacitor is the leakage path from the input source through the input network (C
I
, R
I
+ R
F
) 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.