AD1847 datasheet(24/28 Pages) AD

24 / 28 page

AD1847

REV. B

–24–

Circuits for 2 Vrms line-level inputs and auxiliaries are shown in

Figure 14 and Figure 15. Note that these are divide-by-two

resistive dividers. The input resistor and 560 pF (1000 pF)

capacitor provide the single-pole of antialias filtering required

for the ADCs. If line-level inputs are already at the 1 Vrms levels

expected by the AD1847, 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).

5.1k

560pF

NPO

5.1k

0.33

µF

R_LINE1

R_LINE2

5.1k

560pF

NPO

5.1k

0.33

µF

L_LINE1

L_LINE2

Figure 14. 2 Vrms Line-Level Input Circuit for Line Inputs

3.3k

1000pF

NPO

4.3k

L_AUX1

L_AUX2

µF

R_AUX1

R_AUX2

3.3k

1000pF

NPO

4.3k

µF

Figure 15. 2 Vrms Line-Level Input Circuit for AUX Inputs

Figure 16 illustrates one example of how an electret condenser

microphone requiring phantom power could be connected to

the AD1847. VREF is shown buffered by an op amp; a transistor

like a 2N4124 will also work well 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 k

Ω, R2 = 2 kΩ, and C = 220 pF.

1/2 SSM2135

OR AD820

LEFT

ELECTRET

CONDENSER

MICROPHONE

INPUT

0.33

µF

L_LINE1

L_LINE2

µF

1/2 SSM2135

OR AD820

REF

0.33

µF

R_LINE1

R_LINE2

µF

1/2 SSM2135

OR AD820

REF

RIGHT

ELECTRET

CONDENSER

MICROPHONE

INPUT

Figure 16. “Phantom-Powered” Microphone Input Circuit

Figure 17 shows ac-coupled line outputs. The 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.

47k

µF

L_OUT

47k

µF

R_OUT

Figure 17. Line Output Connections

A circuit for headphone drive is illustrated in Figure 18. Drive is

supplied by +5 V operational amps. The circuit shown ac

couples the headphones to the line output.

SSM2135

20k

470

µF

18k

REF

HEADPHONE

LEFT

20k

470

µF

18k

HEADPHONE

RIGHT

L_OUT

R_OUT

Figure 18. Headphone Drive Connections

Figure 19 illustrates reference bypassing. VREFI should only be

connected to its bypass capacitors.

µF

0.1

µF

REFI

µF

REF

Figure 19. Voltage Reference Bypassing

Figure 20 illustrates signal-path filtering capacitors, L_FILT

and R_FILT. The AD1847 must use 1.0

µF capacitors. The

1.0

µF capacitors required by the AD1847 can be of any type.

1.0

µF

LFILT

1.0

µF

RFILT

Figure 20. External Filter Capacitor Connections

AD1847 Datasheet(PDF) 24 Page - Analog Devices

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