AD1845 Analog Devices, AD1845 Datasheet - Page 34

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

AD1845
however, that dc-coupled inputs are not recommended, as they
provide no performance benefits with the AD1845 architecture.
Furthermore, dc offset differences between multiple dc-
coupled inputs create the potential for “clicks” when changing
the input mux selection.
A circuit for 2 V rms mono, line-level inputs and auxiliaries is
shown in Figure 25 and Figure 26. Note that this is a divide-
by-two resistive dividers considering the codec input imped-
ance. The input resistor and 560 pF (1000 pF) capacitor
provides the single-pole of antialias filtering required for the
ADCs. If line-level inputs are already at the 1 V rms levels ex-
pected by the AD1845, the resistors in parallel with the 560 pF
(1000 pF) capacitors can be omitted. If the application does
not route the AUX2 inputs to the ADCs, then no antialias fil-
tering is required (only the 1 F ac coupling capacitor).
Figure 25. 2 V rms Line-Level Input Circuit for LINE Inputs
The AD1845 codec contains an optional +20 dB gain block to
accommodate condenser microphones. Particular system re-
quirements will depend upon the characteristics of the intended
microphone. Figure 27 illustrates one example of how an elec-
tret condenser mike requiring phantom power could be con-
nected to the AD1845. V
transistor like a 2N4124 will also work fine for this purpose.
Note that if a battery-powered microphone is used, the buffer
and R2s are not needed. The values of R1, R2, and C should
be chosen in light of the mic characteristics and intended gain.
Typical values for these might be R1 = 20 k , R2 = 2 k , and
C = 220 pF.
Figure 26. 2 V rms Line-Level Input Circuit for M_IN and
AUX Inputs
1000pF
1000pF
3.3k
3.3k
NPO
NPO
560pF
560pF
3.3k
3.3k
NPO
NPO
REF
is shown buffered by an op amp; a
4.3k
4.3k
4.3k
4.3k
1µF
1µF
1µF
1µF
R_AUX1
R_AUX2
L_AUX1
L_AUX2
M_IN
L_LINE
R_LINE
–34–
Figure 28 shows ac-coupled line outputs. The resistors are used
to center the output signals around analog ground. If dc-cou-
pling is desired, V
above, if desired.
A circuit for headphone drive is illustrated in Figure 29. Drive is
supplied by +5 V operational amps. The circuit shown ac
couples the headphones to the line output.
Figure 30 illustrates reference bypassing. VREF_F should only
be connected to its bypass capacitors.
Figure 27. “Phantom-Powered” Microphone
Input Circuit
LEFT ELECTRET
RIGHT ELECTRET
MICROPHONE
CONDENSER
MICROPHONE
CONDENSER
INPUT
INPUT
Figure 29. Headphone Drive Connections
R_OUT
L_OUT
V_REF
Figure 28. Line Output Connections
R2
R2
R_OUT
L_OUT
REF
10k
10k
1µF
1µF
could be used with op amps as mentioned
5k
5k
1µF
1µF
SSM-2135
8.66k
8.66k
C
1/2 SSM2135 OR AD820
R1
C
47k
47k
470µF
470µF
R1
1/2 SSM2135
1/2 SSM2135
OR AD820
OR AD820
HEADPHONE
LEFT
HEADPHONE
RIGHT
0.33µF
0.33µF
REV. B
V
L_MIC
R_MIC
V
REF
REF