AD1843 Datasheet, PDF (56/64 Pages) Analog Devices – Serial-Port 16-Bit SoundComm Codec

AD1843
“room” in the frequency domain for a second pole, especially in
modem applications, where the signal bandwidth is 4.2 kHz. A
suggested capacitor value is 120 pF when using the 10K op amp
feedback resistor as shown, for a –3 dB point of approximately
133 kHz. Note that the combined effect of both filters causes a
gain error of approximately 0.1% at 4.2 kHz.
CLOSE TO SOURCE
CLOSE TO AD1843
1µF _10k
10k
SOURCE
1µF _0.9k
10k
10k
CMBUF
1µF
1/2
SSM2135 CMBUF
1/2
SSM2135
LINRN
1000pF ^LINLN
NPO
LINLP
LINRP
Figure 21. Single-Ended to Differential Converter
The AD1843 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 22 illustrates one example of how an elec-
tret condenser mic requiring phantom power could be con-
nected to the AD1843. CMOUT is shown buffered by an op
amp; the current drawn from CMOUT should be as minimal as
possible. 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 in-
tended gain. Typical values for these might be R1 = 20 kΩ,
R2 = 2 kΩ, and C = 220 pF. Figure 22 shows a voltage follower
buffer on the CMOUT signal. The output of this buffer,
CMBUF, can be used in any circuit which needs the common-
mode bias point from the AD1843’s on-chip voltage reference.
The AD820 is a JFET input op amp whose very high imped-
ance inputs present essentially no load on the CMOUT output
from the AD1843.
C
1µF _5k
R1
LEFT ELECTRET
CONDENSER
MICROPHONE
INPUT
R2
1/2 AD820
0.33µF
1/2 SSM2135
OR AD820
MICL
CMOUT
R2
RIGHT ELECTRET
CONDENSER
MICROPHONE
INPUT
CMBUF
1µF _5k
1/2 SSM2135
OR AD820
MICR
0.33µF
R1
LOUT1L
LOUT2LP
LOUT1R
LOUT2RP
1µF
47k
1µF
47k
Figure 23. AD1843 (Single-Ended) Line Output
Connections
For optimal performance, the AD1843 DAC2 output has provi-
sion for a differential configuration. A simple differential-to-
single-ended conversion circuit is shown in Figure 24. The
noise rejection of this circuit is limited by the match of the R_i
and R_f resistors. The resistors should be 1% tolerance compo-
nents. The equation that determines the output voltage is as
follows:
V_O = (R_f/R_i)(V+) – (R_f/R_i)(V–)
For maximum noise immunity, this circuit should be located in
close proximity to where V_O is processed. Another differential
receiver option is the SSM2141, which provides better matching
than a discrete implementation.
LOUT2LN
(V–)
R_i
10pF
R_f
LOUT2RN
(V–)
R_i
10pF
R_f
LOUT2LP
(V+)
R_i
CMBUF
10pF
V_O
1/2
SSM2135
R_f
LOUT2RP
(V+)
CMBUF
R_i
10pF
V_O
1/2
SSM2135
R_f
1/2
SSM2135
1/2
SSM2135
Figure 24. AD1843 Differential-to-Single-Ended Converter
Line Output Connections
A circuit for headphone drive is illustrated in Figure 25. The
AD1843 headphone output is designed to drive loads of 32
ohms or smaller (i.e., higher impedance). If larger loads are
used (e.g., 16 ohms or 8 ohms), the analog output will be dis-
torted for large output signals because of current limiting.
“Walkman”-type headphone impedances are generally around
32 ohms. Telephone handset impedances are typically 150 ohms.
HPOUTL
HPOUTR
C
Figure 22. AD1843 “Phantom-Powered” Microphone
Input Circuit
Connect unused analog inputs to CMBUF or leave them
unconnected; do not connect unused analog inputs to
either analog power or ground!
Figure 23 shows ac-coupled line outputs. The resistors are used
to center the output signals around analog ground. If dc-
coupling is desired, CMOUT could be used with op amps as
mentioned above, if desired.
HPOUTC
Figure 25. AD1843 Headphone Drive Connections
Figure 26 shows an example circuit for the mono output
(MOUT) from the AD1843. The OP279 is a single +5 V supply
op amp which can drive a small 8 ohm or 16 ohm speaker to
modest levels.
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