OP183

Rev. D | Page 14 of 16

LOW VOLTAGE HEADPHONE AMPLIFIERS

Figure 36 shows a stereo headphone output amplifier for the

AD1849 16-bit SoundPort® Stereo Codec device. The

pseudoreference voltage is derived from the common-mode

voltage generated internally by the AD1849, thus providing a

convenient bias for the headphone output amplifiers.

OP183

OP183

OP183

5V

5k

Ω

OPTIONAL

GAIN

1k

Ω

L VOLUME

CONTROL

16

Ω 220μF

47k

Ω

HEADPHONE

LEFT

5V

R VOLUME

CONTROL

5k

Ω

OPTIONAL

GAIN

1k

Ω

16

Ω 220μF

47k

Ω

HEADPHONE

RIGHT

AD1849

20

19

21

LOUT1L

LOUT1R

CMOUT

10

μF

10k

Ω

10k

Ω

10

μF

Figure 36. Headphone Output Amplifier for Multimedia Sound Codec

LOW NOISE MICROPHONE AMPLIFIER FOR

MULTIMEDIA

The OP183 is ideally suited as a low noise microphone preamp

for low voltage audio applications. Figure 37 shows a gain of 100

stereo preamp for the AD1849 16-bit SoundPort Stereo Codec

chip. The common-mode output buffer serves as a phantom

power driver for the microphones.

OP183

OP183

5V

10k

Ω

10k

Ω

AD1849

18

19

17

MINL

MINR

CMOUT

1/2

OP219

5V

100

Ω

100

Ω

10k

Ω

10k

Ω

50

Ω

10

μF

50

Ω

10

μF

20

Ω

20

Ω

RIGHT

ELECTRET

CONDENSER

MIC

INPUT

LEFT

ELECTRET

CONDENSER

MIC

INPUT

Figure 37. Low Noise Stereo Microphone Amplifier for

Multimedia Sound Codec

3 V 50 HZ/60 HZ ACTIVE NOTCH FILTER WITH

FALSE GROUND

To process ac signals, it may be easier to use a false-ground bias

rather than the negative supply as a reference ground. This

would reject the power line frequency interference which can

often obscure low frequency physiological signals, such as heart

rates, blood pressures, EEGs, and ECGs.

Figure 38 shows a 50 Hz/60 Hz active notch filter for

eliminating line noise in patient monitoring equipment. It has

several kilohertz bandwidth and is not sensitive to false-ground

perturbations. The simple false-ground circuit shown achieves

good rejection of low frequency interference using standard off-

the-shelf components.

4

5

3

1

2

A1

8

7

6

OP183

4

1

3

A3

R1

2.67k

Ω

3V

C1

1

μF

R3

2.67k

Ω

C2

1

μF

R4

2.67k

Ω

R2

2.67k

Ω

R5

1.33k

Ω

(2.67k

Ω÷ 2)

C3

1

μF

(1

μF × 2)

R8

1k

Ω

R7

1k

Ω

C5

0.015

μF

R12

70

Ω

R11

10k

Ω

0.75V

C6

1

μF

R10

25k

Ω

C4

1

μF

R9

75k

Ω

3V

R6

10k

Ω

A2

OP183

OP183

Q = 0.75

NOTE:

FOR 50Hz APPLICATIONS

CHANGE R1–R4 TO 3.1

Ω

AND R5 TO 1.58

Ω (3.16Ω÷ 2).

Figure 38. 3 V Supply 50 Hz/60 Hz Notch Filter with Pseudo Ground

Amplifier A3 biases A1 and A2 to the middle of their input

common-mode range. When operating on a 3 V supply, the

center of the common-mode range of the OP183 is 0.75 V. This

notch filter effectively squelches 60 Hz pickup at a filter Q of

0.75. To reject 50 Hz interference, change the resistors in the

twin-T section (R1 through R5) from 2.67 kΩ to 3.16 kΩ.

The filter section uses OP183 op amps in a twin-T

configuration whose frequency selectivity is very sensitive to

the relative matching of the capacitors and resistors in the twin-

T section. Mylar is the material of choice for the capacitors, and

the relative matching of the capacitors and resistors determines

the filter’s pass-band symmetry. Using 1% resistors and 5%

capacitors produces satisfactory results.