Diodes PAM8303C - Manual (Page 10)

PAM8303C
Document number: DSxxxxx Rev. 1 - 2
10 of 17
www.diodes.com
June 2013
© Diodes Incorporated
PAM8303C
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 22 μH inductors are used in series with load resistor to emulate the small speaker for efficiency measurement.
Input Resistance (RI)
The input resistors (R I ) set the gain of the amplifier according to Equation 1.





 

V
V
R
k 150 x 2
Gain
I
Resistor matching is very important in fully differential amplifiers. The balance of the output on the reference voltage depends on matched ratios
of the resistors. CMRR, PSRR, and cancellation of the second harmonic distortion diminish if resistor mismatch occurs. Therefore, it is
recommended to use 1% tolerance resistors or better to keep the performance optimized. Matching is more important than overall tolerance.
Resistor arrays with 1% matching can be used with a tolerance greater than 1%.
Place the input resistors very close to the PAM8303C to limit noise injection on the high impedance nodes.
For optimal performance the gain should be set to 2X (R
I
= 150k) or lower. Lower gain allows the PAM8303C 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, higher value of R I 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, Ci and the minimum input impedance R
I
form is a high-pass filter with the corner frequency determined in 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. For example, when R
i
is 150k Ω and
the specification calls for a flat bass response are down to 150Hz. Equation is reconfigured as followed:
F R 2
1
C
CI I
I 

When input resistance variation is considered, the C
I
is 7nF, so one would likely choose a value of 10nF. 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.
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 is held at V DD /2, which is likely higher than the source DC level.
Please note that it is important to confirm the capacitor polarity in the application.