BA3121F

7/12

TSZ02201-0C2C0EZ00390-1-2

© 2015 ROHM Co., Ltd. All rights reserved.

13.Nov.2015 Rev.001

www.rohm.com

TSZ22111・15・001

Application Information

1.

Circuit Operation

Figure 13. Flow of Noise in Car Audio Systems

Car audio systems are grounded to the car body. For

this reason, electrical noise generated by the car

electrical system can enter the power amplifier input

through the chassis and become audible.

BA3121F utilizes the common-mode rejection

characteristics of an operational amplifier to eliminate

this noise. Without BA3121F noise enters the power

amplifier inputs directly. With BA3121F, the CMRR of

operational amplifiers 1-A and 2-A eliminates the noise.

Principle of noise elimination:

Figure 14. The Principle of Noise Rejection

With BA3121F, the elimination level of

the noise is expressed as:

Therefore, CMRR

≥ 41dB can be guaranteed.

From

① and ②





2

4

3

1

3

2

4

1

2

1

1

2

2

4

3

4

1

2

1

1

1

2

)

(

-

-

-

)

(

-

e

R

R

R

R

R

R

R

e

e

R

R

e

R

R

R

R

R

R

e

R

R

e

O























and tolerances in the operational amplifier, a noise voltage is generated.





i

O

i

V

R

R

R

e

R

R

e

e

R

R

R

V

















1

2

1

1

1

2

2

4

3

4

①

②