AD1846 Analog Devices, AD1846 Datasheet - Page 25

• Manufacturer Part Number: AD1846
• Description: Low Cost Parallel-port 16-bit Soundport Stereo Codec
• Manufacturer: Analog Devices

REV. A
APPLICATIONS CIRCUITS
The AD1846 Stereo Codec has been designed to require a mini-
mum of external circuitry. The 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.
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. This 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.
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.) The 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. Thus, 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. The V
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.
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.
The 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.
The 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 17. 2 V rms Line-Level Input Circuits
560pF
5.1k
560pF
5.1k
NPO
NPO
REF
pin will be at this intermediate
5.1k
5.1k
0.33 F
0.33 F
R_LINE
R_AUX1
R_AUX2
L_LINE
L_AUX1
L_AUX2
–25–
Figure 18 illustrates one example of how an electret condenser
mike requiring phantom power could be connected to the
AD1846. V
a 2N4124 will also work fine for this purpose.
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 R
chosen in light of the mic characteristics and intended gain.
Typical values for these might be R
C = 220 pF.
Figure 19 shows ac-coupled line outputs. The resistors are used
to center the output signals around analog ground. If
dc-coupling is desired, V
mentioned previously.
A circuit for headphone drive is illustrated in Figure 20. Drive is
supplied by +5 V operational amplifiers. The circuit shown ac
couples the line output to the headphones.
Figure 18. “Phantom-Powered” Microphone Input Circuit
LEFT ELECTRET
RIGHT ELECTRET
MICROPHONE
CONDENSER
MICROPHONE
CONDENSER
L_OUT
R_OUT
2
s are not needed. The values of R
V
REF
Figure 20. Headphone Drive Connections
INPUT
INPUT
REF
Figure 19. Line Output Connections
is shown buffered by an op amp; a transistor like
L_OUT
R_OUT
R
R
20k
20k
2
2
1/2 SSM-2135
REF
1 F
1 F
OR AD820
SSM-2135
1 F
1 F
could be used with op amps as
5.1k
18k
5.1k
18k
1
47k
47k
= 20 k , R
470 F
470 F
R
R
C
C
1
1
1
, R
1/2 SSM-2135
1/2 SSM-2135
OR AD820
OR AD820
2
s, and C should be
HEADPHONE
LEFT
HEADPHONE
RIGHT
AD1846
2
0.33 F
0.33 F
= 2 k , and
L_MIC
R_MIC
V
V
REF
REF