AD1885
MICROPHONE CONNECTIONS
The AD1885 contains an internal microphone preamp with 20 dB gain; in most cases a direct microphone connection as shown in
Figure 14 is adequate. If the microphone level is too low, an external preamp can be added as shown in Figure 15. In either case the
microphone bias can be derived from the Codec’s internal reference (VREFOUT) using a 2.2 kΩ resistor. For the preamp circuit,
the VREFOUT signal can also provide the midpoint bias for the amplifier.
To meet the PC99 1.0A requirements, the MIC signal should be placed on the microphone jack tip and the bias on the ring. This
configuration supports electret microphones with three conductor plugs, as well as dynamic microphones with two conductor plugs
(ring and sleeve shorted together).
Additional filtering may be required to limit the microphone response to the audio band of interest.
EMC
COMPONENTS
L1 600Z
L2 600Z
J1
1
2
3
4
5
AC-COUPLING
MIC BIAS
C2
470pF
TO CODEC MIC1 OR MIC2 INPUT
FROM CODECVREFOUT
C1
470pF
MIC INPUT
Figure 14. Recommended Microphone Input Connections
PREAMP
EMC
COMPONENTS
L1 600Z
J1
1
2
3
4
5
AC-COUPLING
AVDD
C2
470pF
4
L2 600Z
C1
U1
TO CODEC MIC1 OR MIC2 INPUT
FROM CODECVREFOUT
470pF
AD8531
MIC INPUT
MIC BIAS
Figure 15. Microphone with Additional External Preamp (20 dB Gain)
LINE OUTPUT CONNECTIONS
The AD1885 Codec provides stereo LINE_OUT signals at a standard 1 V rms level. These signals must be ac-coupled before they
can be connected to an external load. After the ac-coupling, a minimal resistive load is recommend to keep the capacitors properly
biased and reduce click and pop when plugging stereo equipment into the output jack. The capacitor values should be selected to
provide a desired frequency response, taking into account the nominal impedance of the external load. To meet the PC99 specifica-
tion for PCs, testing must be performed with a 10 kΩ load, therefore a 1 µF value is recommended to achieve less than –3 dB
roll-off at 20 Hz.
EMC
COMPONENTS
AC-COUPLING
STEREO LINE_OUT JACK
J1
L2 600Z
FROM CODEC LINE_OUT_R
FROM CODEC LINE_OUT_L
C1
470pF
L1 600Z
C2
470pF
NOTE:
IF AN OUTPUT AMP IS USED,THE
AC-COUPLING CAPVALUESWILL
DEPENDEND ONTHE AMP DESIGN.
Figure 16. Recommended LINE_OUT Connections
REV. 0
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