717
Audio ICs
Ground isolation amplifier
BA3121 / BA3121F / BA3121N
The BA3121,BA3121F and BA3121N are ground isolation amplifiers developed for use in car audio applications. These
ICs efficiently eliminate problems caused by wiring resistance, and remove noise generated by the electrical devices
used in automobiles. The capacitance values of the external capacitors required for the ICs are small to allow compact
and reliable set design.
F
Applications
Car audio systems
F
Features
1) Large capacitors not required
2) High common-mode rejection ratio
(57dB typ. at f = 1kHz).
3) Low noise (V
NO
= 3.5
µ
Vrms Typ.).
4) Low distortion (THD = 0.002% Typ.).
5) Two channels.
F
Absolute maximum ratings (Ta = 25
_
C)
F
Recommended operating conditions (Ta = 25
_
C)
719
Audio ICs
BA3121 / BA3121F / BA3121N
F
Electrical characteristics (unless otherwise noted, Ta = 25
_
C, V
CC
= 12V, f = 1kHz, R
g
= 1.8k
)
V
i
=
R
4
(R
3
)
R
4
)
S
e
2
e
O
=
*
R
1
)
R
2
R
1
S
V
i
R
2
R
1
e
1
)
e
O
=
*
R
1
)
R
2
R
1
S
e
2
R
2
R
1
e
1
)
e
O
=
*
R
1
R
4
*
R
2
R
3
R
1
(R
3
)
R
4
)
S
e
2
R
2
R
1
S
(e
1
*
e
2
)
)
R
4
(R
3
)
R
4
)
S
1
2
721
Audio ICs
BA3121 / BA3121F / BA3121N
F
Circuit operation
Car-audio systems are earthed to the car body, and for
this reason, electrical noise generated by the car elec-
trics can enter the power amplifier input via the chassis,
and become audible.
The BA3121 makes use of the common-mode rejection
characteristics of an operational amplifier to eliminate
this noise. Without the BA3121 noise enters the power
amplifier input directly, when used, the CMMR of opera-
tional amplifiers 1-A and 2-A eliminates the noise.
Principles of noise elimination:
To obtain the output voltage (e
O
)
From
1
and
2
Ideally, if R
1
R
4
= R
2
R
3
, and e
1
= e
2
, the noise voltage will
become zero. However, due to mismatching between the
resistors, difference in the noise voltages (e
1
and e
2
), and
tolerances in the operational amplifier, a noise voltage
does result.
With the BA3121, the elimination level of the noise is ex-
pressed as: CMMR = 20log (e
O
/ e
i
) (e
i
= e
1
= e
2
)
Therefore, CMRR
y
41dB can be guaranteed.
F
Operation notes
(1)
Maintain a ratio of 2: 1 for the values of the capaci-
tors connected to pin 2 (V
m1
) and pin 6 (V
m2
) to keep the
ripple rejection ratio stable. If this ratio is maintained, the
ripple rejection ratio will not vary significantly even if the
capacitance values are halved.
(2)
If the value of the capacitor connected to pin 2 (V
m1
)
in the example is doubled, the bass-region CMMR will be
+6dB, and if it is halved, it will be
*
6dB (see Fig. 16).
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