ADA4075-2

Rev. 0 | Page 18 of 24

BALANCED LINE RECEIVER

Figure 63 depicts a unity-gain balanced line receiver capable of

a high degree of hum rejection. The CMRR is approximately

given by

⎟

⎠

⎞

⎜

⎝

⎛

R2R3

R1R4

10

log

20

Therefore, R1 to R4 should be close-tolerance components to

obtain the best possible CMRR without adjustment. The presence

of A2 ensures that the impedances are symmetric at the two inputs

(unlike many other designs), and, as a bonus, A2 also provides a

complementary output. A3 raises the common-mode input

impedance from about 7.5 kΩ to about 70 kΩ, reducing the

degradation of CMRR due to mismatches in source impedance.

It should be noted that A3 is not in the signal path, and almost any

op amp will work well here. Although it may seem as though the

inverting output should be noisier than the noninverting one,

they are in fact symmetric at about −111 dBV (20 kHz bandwidth).

Sometimes an overall gain of ½ is desired to provide an extra

6 dB of differential input headroom. This can be attained by

reducing R3 and R4 to 5 kΩ and increasing R9 to 22 kΩ.

R5

5kΩ

OUT–

OUT+

IN–

*A3 REDUCES THE DEGRADATION OF CMRR

(SEE THE BALANCED LINE RECEIVER SECTION FOR MORE DETAILS).

C1

22µF

(NON-POLAR)

IN+

R4

10kΩ

1/2

ADA4075-2

R3

10kΩ

R6

5kΩ

R1

5kΩ

R2

5kΩ

C2

50pF

R9

11kΩ

A3*

R8

5.6kΩ

R7

5.6kΩ

1/2

A1

A2

ADA4075-2

C3

50pF

R10

11kΩ

Figure 63. Balanced Line Receiver