AD1847
Circuits for 2 Vrms line-level inputs and auxiliaries are shown in
Figure 14 and Figure 15. Note that these are divide-by-two
resistive dividers. T he input resistor and 560 pF (1000 pF)
capacitor provide the single-pole of antialias filtering required
for the ADCs. If line-level inputs are already at the 1 Vrms levels
expected by the AD1847, 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
filtering is required (only the 1 µF ac coupling capacitor).
Figure 17 shows ac-coupled line outputs. T he resistors are
used to center the output signals around analog ground. If
dc-coupling is desired, VREF could be used with op amps as
mentioned previously.
1µF
L_OUT
47k
1µF
R_OUT
0.33 µF
5.1k
L_LINE1
L_LINE2
47k
560pF
5.1k
NPO
Figure 17. Line Output Connections
0.33 µF
5.1k
R_LINE1
R_LINE2
A circuit for headphone drive is illustrated in Figure 18. Drive is
supplied by +5 V operational amps. T he circuit shown ac
couples the headphones to the line output.
560pF
5.1k
NPO
18k
20k
470µF
470µF
L_OUT
Figure 14. 2 Vrm s Line-Level Input Circuit for Line Inputs
HEADPHONE
LEFT
V
SSM2135
18k
REF
1µF
3.3k
L_AUX1
L_AUX2
20k
HEADPHONE
RIGHT
R_OUT
1000pF
4.3k
NPO
1µF
3.3k
R_AUX1
Figure 18. Headphone Drive Connections
R_AUX2
1000pF
4.3k
Figure 19 illustrates reference bypassing. VREFI should only be
connected to its bypass capacitors.
NPO
V
V
REF
REFI
0.1µF
10µF
10µF
Figure 15. 2 Vrms Line-Level Input Circuit for AUX Inputs
Figure 16 illustrates one example of how an electret condenser
microphone requiring phantom power could be connected to
the AD1847. VREF is shown buffered by an op amp; a transistor
like a 2N4124 will also work well for this purpose. Note that if a
battery-powered microphone is used, the buffer and R2s are not
needed. T he values of R1, R2, and C should be chosen in light
of the mic characteristics and intended gain. T ypical values for
these might be R1 = 20 kΩ, R2 = 2 kΩ, and C = 220 pF.
Figure 19. Voltage Reference Bypassing
Figure 20 illustrates signal-path filtering capacitors, L_FILT
and R_FILT . T he AD1847 must use 1.0 µF capacitors. T he
1.0 µF capacitors required by the AD1847 can be of any type.
LFILT
RFILT
C
1.0µF
1.0µF
R1
1µF
5k
0.33µF
L_LINE1
L_LINE2
Figure 20. External Filter Capacitor Connections
1/2 SSM2135
OR AD820
R2
LEFT
VREF
ELECTRET
CONDENSER
MICROPHONE
INPUT
1/2 SSM2135
OR AD820
C
R2
R1
1µF
5k
0.33µF
RIGHT
ELECTRET
CONDENSER
MICROPHONE
INPUT
R_LINE1
R_LINE2
1/2 SSM2135
OR AD820
VREF
Figure 16. “Phantom -Powered” Microphone Input Circuit
–24–
REV. B
ADI [ ADI ]
