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.
00292-036
OP183
OP183
OP183
5V
5kΩ
OPTIONAL
GAIN
1kΩ
V
REF
L VOLUME
CONTROL
16Ω
220μF
47kΩ
HEADPHONE
LEFT
5V
R VOLUME
CONTROL
5kΩ
OPTIONAL
GAIN
1kΩ
V
REF
16Ω
220μF
47kΩ
HEADPHONE
RIGHT
AD1849
20
19
21
LOUT1L
LOUT1R
CMOUT
V
REF
10μF
10kΩ
10kΩ
10μF
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.
00292-037
OP183
OP183
5V
10kΩ
10kΩ
AD1849
18
19
17
MINL
MINR
CMOUT
1/2
OP219
5V
100Ω
100Ω
10kΩ
10kΩ
50Ω
10μF
50Ω
10μF
20Ω
20Ω
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
2.67kΩ
3V
C1
1μF
R3
2.67kΩ
C2
1μF
R4
2.67kΩ
R2
2.67kΩ
R5
1.33kΩ
(2.67kΩ÷ 2)
C3
1μF
(1μF × 2)
R8
1kΩ
R7
1kΩ
C5
0.015μF
R12
70Ω
R11
10kΩ
0.75V
C6
1μF
R10
25kΩ
C4
1μF
R9
75kΩ
3V
V
IN
R6
10kΩ
V
O
00292-038
A2
OP183
OP183
Q = 0.75
NOTE:
FOR 50Hz APPLICATIONS
CHANGE R1–R4 TO 3.1Ω
AND R5 TO 1.58Ω (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.
