lm48861tm National Semiconductor Corporation, lm48861tm Datasheet - Page 10

• Manufacturer Part Number: lm48861tm
• Description: Powerwise Ground-referenced, Ultra Low Noise, Stereo Headphone Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
The -3dB point of the high-pass filter is found using Equation
(2) below.
Where the value of R
resistor selection. In relation to Figure 1, R
= C1 = C2.
The input capacitors can also be used to remove low fre-
quency content from the audio signal. Small speakers can not
reproduce, and may even be damaged by low frequencies.
High-pass filtering the audio signal helps protect the speak-
ers. When the LM48861 is using a single-ended source,
power supply noise on the ground is seen as an input signal.
Setting the high-pass filter point above the power supply noise
frequencies, 217Hz in a GSM phone, for example, filters out
the noise such that it is not amplified and heard on the output.
Capacitors with a tolerance of 10% or better are recommend-
ed for impedance matching and improved CMRR and PSRR.
PCB Layout Guidelines
Minimize trace impedance of the power, ground and all output
traces for optimum performance. Voltage loss due to trace
f = 1 / 2
IN
is selected based on the gain-setting
π
R
IN
C
IN
IN
= R1 = R2, C
(2)
IN
10
resistance between the LM48861 and the load results in de-
creased output power and efficiency. Trace resistance be-
tween the power supply and ground has the same effect as a
poorly regulated supply, increased ripple and reduced peak
output power. Use wide traces for power supply inputs and
amplifier outputs to minimize losses due to trace resistance,
as well as route heat away from the device. Proper grounding
improves audio performance, minimizes crosstalk between
channels and prevents switching noise from interfering with
the audio signal. Use of power and ground planes is recom-
mended.
As described in the Common Mode Sense section, the
LM48861 features a ground sensing feature. On the PCB lay-
out, connect the COM pin (pin D2) directly to the headphone
jack ground and also to the left and right input grounds. This
will help correct any noise or any ground imbalance between
the headphone return, input, and the device ground, therefore
improving audio reproduction.
The charge pump capacitors and traces connecting the ca-
pacitor to the device should be kept away from the input and
output traces to avoid any switching noise injected into the
input or output.