IS31AP2010B
Integrated Silicon Solution, Inc. – www.issi.com
Rev.B, 04/10/2013
10
INPUT CAPACITORS (C
IN
)
The input capacitors and input resistors form a high
pass filter with the corner frequency, f
C
, determined in
Equation (2).
ININ
CR
c
f
2
1
(2)
The value of the input capacitor is important to
consider as it directly affects the bass (low frequency)
performance of the circuit. Speakers in wireless
phones cannot usually respond well to low frequencies,
so the corner frequency can be set to block low
frequencies in this application.
Equation (3) is reconfigured to solve for the input
coupling capacitance.
CIN
IN
fR
C
2
1
(3)
If the corner frequency is within the audio band, the
capacitors should have a tolerance of ±10% or better,
because any mismatch in capacitance causes an
impedance mismatch at the corner frequency and
below.
For a flat low frequency response, use large input
coupling capacitors (1μF). However, in a GSM phone
the ground signal is fluctuating at 217Hz, but the signal
from the codec does not have the same 217Hz
fluctuation. The difference between the two signals is
amplified, sent to the speaker, and heard as a 217Hz
hum.
SUMMING INPUT SIGNALS
Most wireless phones or PDAs need to sum signals at
the audio power amplifier or just have two signal
sources that need separate gain. The IS31AP2010B
makes it easy to sum signals or use separate signal
sources with different gains. Many phones now use the
same speaker for the earpiece and ringer, where the
wireless phone would require a much lower gain for
the phone earpiece than for the ringer. PDAs and
phones that have stereo headphones require summing
of the right and left channels to output the stereo signal
to the mono speaker.
SUMMING TWO DIFFERENTIAL INPUT SIGNALS
Two extra resistors are needed for summing
differential signals (a total of 5 components). The gain
for each input source can be set independently (see
Equations (4) and (5) and Figure 12).
1
1
1502
1
IN
I
O
R
k
Gain
V
V
V
V
(4)
2
2
1502
2
IN
I
O
R
k
Gain
V
V
V
V
(5)
If summing left and right inputs with a gain of 1V/V, use
R
IN1
= R
IN2
= 300kΩ.
If summing a ring tone and a phone signal, set the
ring-tone gain to Gain1 = 2V/V, and the phone gain to
Gain2 = 0.1V/V. The resistor values would be
R
IN1
= 150kΩ, R
IN2
= 3MΩ.
Figure 12 Summing Two Differential Inputs
SUMMING A DIFFERENTIAL INPUT SIGNAL AND A
SINGLE-ENDED INPUT SIGNAL
Figure 13 shows how to sum a differential input signal
and a single-ended input signal. Ground noise may
couple in through IN- with this method. It is better to
use differential inputs. The gain for each input source
can be set independently by Equations (4) and (5). The
corner frequency of the single-ended input is set by
C
IN2
, shown in Equation (6).
CIN
IN
fR
C
2
2
2
1
(6)
To assure that each input is balanced, the
single-ended input must be driven by a low-impedance
source even if the input is not in use. If summing a ring
tone and a phone signal, the phone signal should use
a differential input signal while the ring tone might be
limited to a single-ended signal. Ring-tone gain is set
to Gain1 = 2V/V, and phone gain is set to Gain2 =
0.1V/V, the resistor values would be R
IN1
= 150kΩ, R
IN2
= 3MΩ.
The high pass corner frequency of the single-ended
input is set by C
IN2
. If the desired corner frequency is
less than 20Hz.
So,
Hzk
IN
C
201502
1
2
and
pFC
IN
53
2
