Using the AT89RFD-08 Board
AT89RFD-08 Hardware User Guide
2-21
4382E–MP3–05/06
2.11
FAQ
1. Can the decoupling capacitors for AUDVCM, CBP and AUDVREF be changed?
Lowering the decoupling capacitors increases the noise (due to power supply rejection)
in low frequency (i.e. dividing the capacitor value by 2 increases the noise by 2 in low
frequency). Raising the decoupling capacitors increases the start-up time (due to the
charge of the capacitor through their resistive biasing).
A tolerance value of +/-10% for all these capacitors is recommended (the variation of the
noise performances will be negligible).
2. Is it possible to use a 6.5W load impedance for the PA?
Yes, it is possible to use it at the expense of a THD degraded (some dB) and higher
power dissipation. (25% raised with a 20% lower load impedance). Refer to product
datasheet plot maximum dissipated power versus power supply and impedance load.
3. The capacitors for the Headset 100uF are too big, can I change their value?
These capacitors set the cutting frequency. With 32W load, cutting frequency Fc is:
F
c
=1/(2.PI.R
LOAD
.C)
The effective power is given by the following formula:
P
EFF MAX
= (1.2 / 1.414)
2
/ R
LOAD
= 0.72 / R
LOAD
[W Rms]
Figure 2-21.
Headset Output
Table 2-5.
Cutting Frequency Versus Effective Power
It is possible to add a serial resistor Rs between R
LOAD
and AUDVSS, to increase the
cutting frequency but in this case, it decreases the maximum effective power in the
Load.
HSL
AUDVSS
R
LOAD
C
HSR
Capacitor C (
m
F)
Load Resistance (Ohms)
Cutting Frequency (Hz)
Effective Power (mW Rms)
100 32
50
22.5
22 32
226
22.5
10 32
497
22.5