AD1845 however, that dcoupled inputs are not recommended, as they provide no performance benefits with the AD1845 architecture. Furthermore, dc offset differences between multiple dccoupled inputs create the potential for “clicks” when changing the input mux selection.
C LEFT ELECTRET CONDENSER MICROPHONE INPUT
R2 1/2 SSM2135 OR AD820 C
R1 5kΩ 0.33µF R_MIC
RIGHT ELECTRET CONDENSER MICROPHONE INPUT
1/2 SSM2135 OR AD820 VREF
Figure 27. “Phantom-Powered” Microphone Input Circuit
Figure 28 shows acoupled line outputs. The resistors are used to center the output signals around analog ground. If dcoupling is desired, VREF could be used with op amps as mentioned above, if desired.
L_LINE 560pF NPO
1/2 SSM2135 OR AD820
A circuit for 2 V rms mono, line-level inputs and auxiliaries is shown in Figure 25 and Figure 26. Note that this is a divideby-two resistive dividers considering the codec input impedance. The input resistor and 560 pF (1000 pF) capacitor provides the single-pole of antialias filtering required for the ADCs. If line-level inputs are already at the 1 V rms levels expected by the AD1845, 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). 3.3kΩ
1µF L_OUT 47kΩ
Figure 25. 2 V rms Line-Level Input Circuit for LINE Inputs 1µF
3.3kΩ 1000pF NPO
L_AUX1 L_AUX2 M_IN
Figure 28. Line Output Connections
1µF R_AUX1 R_AUX2
A circuit for headphone drive is illustrated in Figure 29. Drive is supplied by +5 V operational amps. The circuit shown ac couples the headphones to the line output.
Figure 26. 2 V rms Line-Level Input Circuit for M_IN and AUX Inputs
The AD1845 codec contains an optional +20 dB gain block to accommodate condenser microphones. Particular system requirements will depend upon the characteristics of the intended microphone. Figure 27 illustrates one example of how an electret condenser mike requiring phantom power could be connected to the AD1845. VREF is shown buffered by an op amp; a transistor like a 2N4124 will also work fine for this purpose. 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 intended gain. Typical values for these might be R1 = 20 kΩ, R2 = 2 kΩ, and C = 220 pF.
Figure 29. Headphone Drive Connections
Figure 30 illustrates reference bypassing. VREF_F should only be connected to its bypass capacitors.