AD1846
AP P LICATIO NS CIRCUITS
Figure 18 illustrates one example of how an electret condenser
mike requiring phantom power could be connected to the
AD1846. VREF is shown buffered by an op amp; a transistor like
a 2N4124 will also work fine for this purpose.
T he AD1846 Stereo Codec has been designed to require a mini-
mum of external circuitry. T he recommended circuits are shown
in Figures 17 through 25. Analog Devices estimates that the to-
tal cost of all the components shown in these figures, including
crystals but not including connectors, to be less than $10 in the
U.S.A. in 10,000 quantities.
Particular system requirements will depend upon the character-
istics of the intended microphone.
Note that if a battery-powered microphone is used, the buffer
and R2s are not needed. T he values of R1, R2s, 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.
See Figure 1 for an illustration of the connection between the
AD1846 SoundPort Codec and the Industry Standard Architec-
ture (ISA) computer bus, also known as the “PC-AT bus.”
Note that the 74_245 transceiver receives its enable and direc-
tion signals directly from the Codec. Analog Devices recom-
mends using the “slowest” 74_245 adequately fast to meet all
AD1846 and computer bus timing and drive requirements. So
doing will minimize switching transients of the 74_245. T his in
turn will minimize the digital feedthrough effects of the trans-
ceiver when driving the AD1846, which can cause the audio
noise floor to rise.
C
R
1
1µF
5.1k
0.33µF
L_MIC
1/2 SSM-2135
OR AD820
R
R
1/2 SSM-2135
OR AD820
2
V
LEFT ELECTRET
CONDENSER
MICROPHONE
INPUT
REF
Industry-standard compact disc “line-levels” are 2 V rms cen-
tered around analog ground. (For other audio equipment, “line
level” is much more loosely defined.) T he AD1846 SoundPort
is a +5 V only powered device. Line level voltage swings for the
AD1846 are defined to be 1 V rms for a sine wave ADC input
and 0.707 V rms for a sine wave DAC output. T hus, 2 V rms
input analog signals must be attenuated and either centered
around the reference voltage intermediate between 0 V and
+5 V or ac-coupled. T he VREF pin will be at this intermediate
voltage, nominally 2.25 V. It has limited drive but can be used
as a voltage offset to an op amp input. Note, however, that
dc-coupled inputs are not recommended, as they provide no
performance benefits with the AD1846 architecture. Further-
more, dc offset differences between multiple dc-coupled inputs
create the potential for “clicks” when changing the input mux
selection.
C
2
R
1
1µF
5.1k
0.33µF
R_MIC
RIGHT ELECTRET
CONDENSER
MICROPHONE
INPUT
1/2 SSM-2135
OR AD820
V
REF
Figure 18. “Phantom -Powered” Microphone Input Circuit
Figure 19 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
A circuit for 2 V rms line-level inputs and auxiliaries is shown in
Figure 17. Note that this is a divide-by-two resistive divider.
T he input resistor and 560 pF capacitor provides the single-pole
of anti-alias filtering required for the ADCs. If line-level inputs
are already at the 1 V rms levels expected by the AD1846, the
resistors in parallel with the 560 pF capacitors can be omitted.
47k
1µF
R_OUT
47k
T he circuit shown in Figure 17 will produce gain/attenuation
step sizes for the auxiliary inputs which are a function of the
programmed gain/attenuation.
Figure 19. Line Output Connections
0.33µF
5.1k
L_LINE
A circuit for headphone drive is illustrated in Figure 20. Drive is
supplied by +5 V operational amplifiers. T he circuit shown ac
couples the line output to the headphones.
L_AUX1
5.1k
560pF
NPO
L_AUX2
18k
0.33µF
5.1k
R_LINE
R_AUX1
R_AUX2
20k
470µF
L_OUT
HEADPHONE
LEFT
5.1k
560pF
NPO
V
SSM-2135
REF
470µF
HEADPHONE
RIGHT
20k
Figure 17. 2 V rm s Line-Level Input Circuits
R_OUT
18k
Figure 20. Headphone Drive Connections
REV. A
–25–
ADI [ ADI ]
