IS31AP2010B
Integrated Silicon Solution, Inc. – www.issi.com
Rev.B, 04/10/2013
9
APPLICATION INFORMATION
FULLY DIFFERENTIAL AMPLIFIER
The IS31AP2010B is a fully differential amplifier with
differential inputs and outputs. The fully differential
amplifier consists of a differential amplifier and a
common mode amplifier. The differential amplifier
ensures that the amplifier outputs a differential voltage
on the output that is equal to the differential input times
the gain. The common-mode feedback ensures that
the common-mode voltage at the output is biased
around VCC/2 regardless of the common-mode
voltage at the input. The fully differential IS31AP2010B
can still be used with a single-ended input; however,
the IS31AP2010B should be used with differential
inputs when in a noisy environment, like a wireless
handset, to ensure maximum noise rejection.
ADVANTAGES OF FULLY DIFFERENTIAL
AMPLIFIERS
The fully differential amplifier does not require a
bypass capacitor. This is because any shift in the
mid-supply affects both positive and negative channels
equally and cancels at the differential output.
GSM handsets save power by turning on and shutting
off the RF transmitter at a rate of 217Hz. The
transmitted signal is picked-up on input and output
traces. The fully differential amplifier cancels the signal
much better than the typical audio amplifier.
COMPONENT SELECTION
Figure 10 shows the IS31AP2010B with differential
inputs and input capacitors, and Figure 11 shows the
IS31AP2010B with single-ended inputs. Differential
inputs should be used whenever possible because the
single-ended inputs are much more susceptible to
noise.
OUT+
GND
IS31AP2010B
B1,B2
A3
C3
VCC
OUT-
C
S
1 F
VBattery
C1
IN-
IN+
A1
0.1 F
A2,B3
SDB
C2
100k
CIN+
0.1 F
C
IN-
0.1 F
R
IN+
150k
RIN-
150k
Differential
Input
Shutdown
Control
Figure 10 Differential Input
Figure 11 Single-Ended Input
INPUT RESISTORS (R
IN
)
The input resistors (R
IN
) set the gain of the amplifier
according to Equation (1).
IN
F
R
R
ain
2
G
V
V
(1)
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
IS31AP2010B to limit noise injection on the
high-impedance nodes.
For optimal performance the gain should be set to
2V/V or lower. Lower gain allows the IS31AP2010B to
operate at its best, and keeps a high voltage at the
input making the inputs less susceptible to noise.
DECOUPLING CAPACITOR (C
S
)
The IS31AP2010B is a high performance Class-D
audio amplifier that requires adequate power supply
decoupling to ensure the efficiency is high and total
harmonic distortion (THD) is low. For higher frequency
transients, spikes, or digital hash on the line, a good
low equivalent-series-resistance (ESR) ceramic
capacitor, typically 1μF, placed as close as possible to
the device VCC lead works best. Placing this
decoupling capacitor close to the IS31AP2010B is very
important for the efficiency of the Class-D amplifier,
because any resistance or inductance in the trace
between the device and the capacitor can cause a loss
in efficiency. For filtering lower frequency noise signals,
a 10μF or greater capacitor placed near the audio
power amplifier would also help, but it is not required in
most applications because of the high PSRR of this
device.
