AD1846 datasheet(25/28 Pages) AD

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AD1846

REV. A

–25–

APPLICATIONS CIRCUITS

The AD1846 Stereo Codec has been designed to require a mini-

mum of external circuitry. The recommended circuits are shown

in Figures 17 through 25. Analog Devices estimates that the to-

tal cost of all the components shown in these figures, including

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

AD1846 SoundPort Codec and the Industry Standard Architec-

ture (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

AD1846 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 AD1846, 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 AD1846 SoundPort

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

AD1846 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 voltage offset to an op amp input. Note, however, that

dc-coupled inputs are not recommended, as they provide no

performance benefits with the AD1846 architecture. Further-

more, 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 AD1846, the

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

The circuit shown in Figure 17 will produce gain/attenuation

step sizes for the auxiliary inputs which are a function of the

programmed gain/attenuation.

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. 2 V rms Line-Level Input Circuits

Figure 18 illustrates one example of how an electret condenser

mike requiring phantom power could be connected to the

AD1846. VREF is shown buffered by an op amp; a transistor like

a 2N4124 will also work fine for this purpose.

Particular system requirements will depend upon the character-

istics of the intended microphone.

Note that if a battery-powered microphone is used, the buffer

and R2s are not needed. The values of R1, R2s, and C should be

chosen in light of the mic characteristics and intended gain.

Typical values for these might be R1 = 20 k

Ω, R

2 = 2 k

Ω, 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. “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. 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. Headphone Drive Connections

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