ba3121f ROHM Co. Ltd., ba3121f Datasheet - Page 6

• Manufacturer Part Number: ba3121f
• Description: Ground Isolation Amplifier
• Manufacturer: ROHM Co. Ltd.

Circuit operation
Car audio systems are earthed to the car body, and for
this reason, electrical noise generated by the car electrics
can enter the power amplifier input via the chassis,
and become audible.
The BA3121F makes use of the common-mode rejection
characteristics of an operational amplifier to eliminate
this noise. Without the BA3121F, noise enters the power
amplifier input directly. When used, the CMMR of operational
amplifiers 1-A and 2-A eliminates the noise.
Principles of noise elimination:
To obtain the output voltage (e
Ideally, if R
become zero. However, due to mismatching between the
resistors, difference in the noise voltages (e
tolerances in the operational amplifier, a noise voltage
exists.
From ① and ②
1
R
4
= R
2
R
3
, and e
1
= e
O
)
2
, the noise voltage will
Fig.14 Flow of noise in car audio systems
1
and e
2
), and
6/8
With the BA3121F, the elimination level of the noise is
expressed
as: CMMR = 20log (e
Therefore, CMRR ≧ 41dB can be guaranteed.
Fig.15 The principle of noise rejection
0
/e
I
)(e
I
= e
1
= e
2
)