AD1848 datasheet(25/28 Pages) AD

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AD1848K

REV. 0

–25–

APPLICATIONS CIRCUITS

The AD1848K Stereo Codec has been designed to require a

minimum of external circuitry. The recommended circuits are

shown in Figures 17 through 25. Analog Devices estimates that

the total cost of all the components shown in these figures, in-

cluding 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

AD1848K SoundPort Codec and the Industry Standard Archi-

tecture (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

AD1848K 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 AD1848K, 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 AD1848K SoundPort

is a +5 V only powered device. Line level voltage swings for the

AD1848K 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 VREF pin will be at this intermediate

voltage, nominally 2.25 V. It has limited drive but can be used

as a dc bias to an op amp input. Note, however, that dc-coupled

inputs are not recommended, as they provide no performance

benefits with the AD1848K architecture. Furthermore, 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 AD1848K, the

resistors in parallel with the 560 pF capacitors can be omitted.

5.1k

560pF

L_LINE

L_AUX1

L_AUX2

0.33

µF

NPO

5.1k

560pF

R_LINE

R_AUX1

R_AUX2

0.33

µF

NPO

Figure 17. AD1848K 2 V rms Line-Level Input Circuit

The AD1848K 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 18 illustrates one example of how an elec-

tret condenser mike requiring phantom power could be con-

nected to the AD1848K. 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.

5.1k

µF

1/2 SSM-2135

OR AD820

L_MIC

REF

LEFT ELECTRET

CONDENSER

MICROPHONE

INPUT

5.1k

µF

R_MIC

RIGHT ELECTRET

CONDENSER

MICROPHONE

INPUT

1/2 SSM-2135

OR AD820

1/2 SSM-2135

OR AD820

REF

0.33

µF

0.33

µF

Figure 18. AD1848K "Phantom-Powered" Microphone

Input Circuit

Figure 19 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.

µF

47k

L_OUT

µF

47k

R_OUT

Figure 19. AD1848K Line Output Connections

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.

HEADPHONE

LEFT

HEADPHONE

RIGHT

L_OUT

R_OUT

470

µF

20k

SSM-2135

REF

18k

470

µF

20k

Figure 20. AD1848K Headphone Drive Connections

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