AD1843JS datasheet(56/64 Pages) AD

56 / 64 page

REV. 0

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AD1843

“room” in the frequency domain for a second pole, especially in

modem applications, where the signal bandwidth is 4.2 kHz. A

suggested capacitor value is 120 pF when using the 10K op amp

feedback resistor as shown, for a –3 dB point of approximately

133 kHz. Note that the combined effect of both filters causes a

gain error of approximately 0.1% at 4.2 kHz.

1µF

0.9k

LINRN

LINLN

10k

1/2

SSM2135

10k

SOURCE

10k

1/2

SSM2135

1µF

1000pF

NPO

LINLP

LINRP

CLOSE TO SOURCE

CLOSE TO AD1843

1µF

CMBUF

Figure 21. Single-Ended to Differential Converter

The AD1843 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 22 illustrates one example of how an elec-

tret condenser mic requiring phantom power could be con-

nected to the AD1843. CMOUT is shown buffered by an op

amp; the current drawn from CMOUT should be as minimal as

possible. 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 in-

tended gain. Typical values for these might be R1 = 20 k

Ω,

R2 = 2 k

Ω, and C = 220 pF. Figure 22 shows a voltage follower

buffer on the CMOUT signal. The output of this buffer,

CMBUF, can be used in any circuit which needs the common-

mode bias point from the AD1843’s on-chip voltage reference.

The AD820 is a JFET input op amp whose very high imped-

ance inputs present essentially no load on the CMOUT output

from the AD1843.

1/2 SSM2135

OR AD820

1µF

CMOUT

MICL

LEFT ELECTRET

CONDENSER

MICROPHONE

INPUT

RIGHT ELECTRET

CONDENSER

MICROPHONE

INPUT

0.33µF

1/2 AD820

1/2 SSM2135

OR AD820

1µF

MICR

0.33µF

CMBUF

Figure 22. AD1843 “Phantom-Powered” Microphone

Input Circuit

Connect unused analog inputs to CMBUF or leave them

unconnected; do not connect unused analog inputs to

either analog power or ground!

Figure 23 shows ac-coupled line outputs. The resistors are used

to center the output signals around analog ground. If dc-

coupling is desired, CMOUT could be used with op amps as

mentioned above, if desired.

LOUT1R

LOUT2RP

47k

1µF

LOUT1L

LOUT2LP

47k

1µF

Figure 23. AD1843 (Single-Ended) Line Output

Connections

For optimal performance, the AD1843 DAC2 output has provi-

sion for a differential configuration. A simple differential-to-

single-ended conversion circuit is shown in Figure 24. The

noise rejection of this circuit is limited by the match of the Ri

and Rf resistors. The resistors should be 1% tolerance compo-

nents. The equation that determines the output voltage is as

follows:

VO = (Rf/Ri)(V+) – (Rf/Ri)(V–)

For maximum noise immunity, this circuit should be located in

close proximity to where VO is processed. Another differential

receiver option is the SSM2141, which provides better matching

than a discrete implementation.

1/2

SSM2135

1/2

SSM2135

LOUT2LN

(V–)

10pF

LOUT2LP

(V+)

CMBUF

1/2

SSM2135

1/2

SSM2135

LOUT2RN

(V–)

10pF

LOUT2RP

(V+)

CMBUF

Figure 24. AD1843 Differential-to-Single-Ended Converter

Line Output Connections

A circuit for headphone drive is illustrated in Figure 25. The

AD1843 headphone output is designed to drive loads of 32

ohms or smaller (i.e., higher impedance). If larger loads are

used (e.g., 16 ohms or 8 ohms), the analog output will be dis-

torted for large output signals because of current limiting.

“Walkman”-type headphone impedances are generally around

32 ohms. Telephone handset impedances are typically 150 ohms.

HPOUTL

HPOUTR

HPOUTC

Figure 25. AD1843 Headphone Drive Connections

Figure 26 shows an example circuit for the mono output

(MOUT) from the AD1843. The OP279 is a single +5 V supply

op amp which can drive a small 8 ohm or 16 ohm speaker to

modest levels.

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