AD1845 Analog Devices, AD1845 Datasheet - Page 35

• Manufacturer Part Number: AD1845
• Description: Parallel-port 16-Bit Soundport Stereo Codec
• Manufacturer: Analog Devices

REV. B
Figure 31 illustrates signal-path filtering capacitors, L_FILT
and R_FILT. The AD1845 must use 1.0 F capacitors; the
AD1845 will not perform properly with 1000 pF capacitors.
The 1.0 F capacitors required by the AD1845 can be of any
type.
The crystal shown in the crystal connection circuitry of Fig-
ure 32 should be 24.576 MHz, fundamental-mode and parallel-
tuned. Note that using the exact data sheet frequencies is not re-
quired and that external clock sources can be used to overdrive
the AD1845’s internal oscillators. (See the description of the
CFS2:0 control bits above.) If using an external clock source,
apply it to the crystal input pins while leaving the crystal output
pins unconnected. Attention should be paid to providing low jit-
ter external input clocks.
Note: XTAL2I and XTAL2O, are not used in the AD1845.
Analog Devices also recommends a pull-down resistor for
PWRDWN.
Good, standard engineering practices should be applied for
power-supply decoupling. Decoupling capacitors should be
placed as close as possible to package pins. If a separate analog
power supply is not available, we recommend the circuit shown
in Figure 33 for using a single +5 V supply. Ferrite beads suffice
for the inductors shown. This circuitry should be as close to the
supply pins as is practical.
SUPPLY
Figure 33. Recommended Power Supply Bypassing
+5V
Figure 31. External Filter Capacitor Connections
Figure 30. Voltage Reference Bypassing
V
REF_F
0.1µF
FERRITE/INDUCTOR
Figure 32. Crystal Connections
INDUCTOR
10µF
FERRITE/
0.1µF
20–64pF
XTAL1I
1.0µF
1µF
L_FILT
0.1µF
1µF
1.6
24.576 MHz
V
DD
1.0µF
0.1µF
V
DD
R_FILT
XTAL1O
V
0.1µF
1µF
V
REF
DD
20–64pF
0.1µF
0.1µF
V
DD
V
DD
0.1µF
10µF
0.1µF
V
V
DD
DD
V
CC
0.1µF
0.1µF
V
CC
–35–
GROUNDING AND LAYOUT
Analog Devices recommends a split ground plane as shown in
Figure 34. The analog plane and the digital plane are connected
directly under the AD1845. Splitting the ground plane directly
under the SoundPort Codec is optimal because analog pins will
be located above the analog ground plane and digital pins will
be located directly above the digital ground plane for the best
isolation.
Other schemes may also yield satisfactory results. If the split
ground plane recommended here is not possible, the AD1845
should be entirely over the analog ground plane with the op-
tional 74_245 transceiver over the digital plane.
Some manufacturers of compatible devices differentiate between
digital supply pins used to power internal logic and digital sup-
ply pins used to power the ISA bus driver. Their recommended
layout suggests connecting the internal logic supply pins to the
analog supply. A potential problem can occur if the layout con-
nects digital supply pins to the analog supply. Connecting some
of the digital supply pins to one supply and some of the digital
supply pins to a different supply can create an internal short be-
tween the two different +5 V supplies.
Analog Devices recommends that all digital pins be driven from
the same supply. A common technique to achieve maximum
performance is to use a +5 V regulator to power the analog side
of the codec from the PC’s +12 V supply line, while the stan-
dard PC +5 V supply line powers the entire digital side of the
codec. The separate supplies provide noise isolation for the ana-
log side of the codec, and maximize performance of the
AD1845.
DIGITAL
GROUND
PLANE
AD1845
PLCC
Figure 34. Recommended Ground Plane
GNDD
GNDD
44
25
R_AUX2
43
26
R_FILT
GROUND
ANALOG
PLANE
DIGITAL
GROUND
PLANE
AD1845
TQFP
NC
NC
52
24
AD1845
NC
51
25
R_FILT
GROUND
ANALOG
PLANE