I
2
C Signal Generation Board and Software
(Continued)
20195907
Figure 3. The LM4681 Demonstration Board Schematic
The LM4681 program includes controls for the amplifier’s
volume control, shutdown, mute, and which serial interface
(I2C or SPI) is used. The control program’s on-screen user
interface is shown in Figure 3.
The
DEFAULT
button is used to return the LM4681 to its
power-on reset state (minimum volume setting, mute on, and
shutdown on).
The LM4681’s stereo
VOLUME CONTROL
has 32 steps
and a gain range of –48dB to +30dB (Class D) or –65dB to
+13dB (Class AB headphone). It is controlled using the slider
located at the top of the program’s window. Each time the
slider is moved from one tick mark to another, the program
updates the amplifier’s volume control.
The
POWER
control has two radio buttons. When OFF is
selected, the LM4681 is placed in micropower shutdown.
When ON is selected, the LM4681 is fully operational.
Use the program’s
MUTE
section to mute the amplifier’s
output (Class D and AB headphone simultaneously). Mute
ON mutes the amplifier outputs. Mute OFF unmutes the
amplifier outputs and returns the output signal level to that
set by the VOLUME CONTROL.
The last section of the software’s interface is the
ADDRESS
BIT
. This bit can be set to 1 or 0. The software will force the
I
2
C interface board to apply a logic low or logic high to the
LM4681’s ADR pin according to the radio button that is
selected. The LM4681 will respond to either of the ad-
dresses selected in the software’s Address Bit control.
PCB Layout Guidelines
This section provides general practical guidelines for PCB
layouts that use various power and ground traces. Designers
should note that these are only "rule-of-thumb" recommen-
dations and the actual results are predicated on the final
layout.
POWER AND GROUND CIRCUITS
Star trace routing techniques (returning individual traces
back to a central point rather than daisy chaining traces
together in a serial manner) can have a major positive im-
pact on low-level signal performance. Star trace routing re-
fers to using individual traces that radiate from a signal point
to feed power and ground to each circuit or even device. This
technique may require greater design time, but should not
increase the final price of the board.
AVOIDING TYPICAL DESIGN/LAYOUT PROBLEMS
Avoid ground loops or running digital and analog traces
parallel to each other (side-by-side) on the same PCB layer.
When traces must cross over each other, do so at 90 de-
grees. Running digital and analog traces at 90 degrees to
each other from the top to the bottom side as much as
possible will minimize capacitive noise coupling and
crosstalk.
AN-1488
www.national.com
5