AD1845 datasheet(34/40 Pages) AD

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AD1845

–34–

REV. C

4.3k

1 F

3.3k

560pF

NPO

L_LINE

R_LINE

4.3k

1 F

3.3k

560pF

NPO

Figure 27. 2 V rms Line-Level Input Circuit for LINE Inputs

4.3k

1 F

3.3k

1000pF

NPO

4.3k

1 F

L_AUX1

L_AUX2

M_IN

1000pF

NPO

R_AUX1

R_AUX2

3.3k

Figure 28. 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 re-

quirements will depend upon the characteristics of the intended

microphone. Figure 29 illustrates one example of how an elec-

tret condenser mike requiring phantom power could be con-

nected 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 k

Ω, R2 = 2 kΩ, and

C = 220 pF.

L_MIC

VREF

1 F

1/2 SSM2135 OR AD820

1/2 SSM2135

OR AD820

R_MIC

LEFT ELECTRET

CONDENSER

MICROPHONE

INPUT

RIGHT ELECTRET

CONDENSER

MICROPHONE

INPUT

0.33 F

1/2 SSM2135

OR AD820

0.33 F

VREF

Figure 29. “Phantom-Powered” Microphone Input

Circuit

In the Expanded Mode, MODE2, the AD1845 can be pro-

grammed to change the sample rate selection in 1 Hz incre-

ments “on the fly” and without entering the Mode Change

Enable Sequence. The following sequence applies to the

AD1845 in MODE2 only:

• Enable the Frequency Select Register by setting FREN to 1.

• Change the Lower and Upper Frequency Select Register,

FU7:0 and FL7:0.

APPLICATIONS CIRCUITS

The AD1845 Stereo Codec has been designed to require a

minimum of external circuitry. The recommended circuits are

shown in Figures 27 through 35.

See Figure 1 for an illustration of the connection between the

AD1845 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

AD1845 and computer bus timing and drive requirements. So

doing will minimize switching transients of the 74_245. This in

turn will minimize the digital feed through effects of the trans-

ceiver when driving the AD1845, which can cause the audio

noise floor to rise. In most applications, the 74_245 can be

omitted and the AD1845 connected directly ISA bus taking

advantage of the AD1845’s built-in 16 mA drivers.

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 AD1845 SoundPort

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

AD1845 are defined to be 1 V rms for a sine wave ADC input

and user selectable 0.707 V rms or 1 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 datum to an op amp

input. Note, however, that dc-coupled inputs are not recom-

mended, as they provide no performance benefits with the

AD1845 architecture. Furthermore, dc offset differences be-

tween multiple dc-coupled inputs create the potential for

“clicks” when changing the input mixer selection.

A circuit for 2 V rms mono, line-level inputs and auxiliaries is

shown in Figure 27 and Figure 28. Note that this is a divide-

by-two resistive dividers considering the codec input imped-

ance. 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).

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