Since R3=R4 in the THAT 1240, this equation

can be simplified to

a

In

R

=

++

()

ef

2

The output voltage would then be

Vout

a

R

R

=´ +

()

1

2

1

Since R2=R1, combining equations results in

Vout

In

R

=+ +

()

ef

Figure 6 shows a convenient method of driving a

typical audio ADC with balanced inputs. This circuit

accepts

+24

dBu

in,

and

using

a

pair

of

THAT 1246s, the signal level between their respective

outputs is +24 dBu. An attenuator network brings

the signal down another 24 dB while attenuating the

noise of the line receivers as well.

The output noise of a THAT 1246 is -109 dBu,

and since there are two of them, the total noise level

going into the resistive pad will be -106 dBu.

The

pad reduces the noise level to -130 dBu at the input

to the ADC. The noise density resulting from the line

receivers will therefore be

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com

Rev. 04/29/02

Page 5

Preliminary Information

In Hi

In Lo

AIN- to ADC

AIN+ to ADC

R8

2k10

R9

249R

R19

2k10

C4

6n8

+24 dBu In

Vout

6

In+

3

In-

2

U1

THAT

1246

Vout

6

In+

3

In-

2

U2

3

2

1

U3A

4570

1/2 Vref of ADC

+24 dBu

Zero dBu Out

Ref

Sense

5

1

Ref

Sense

5

1

THAT

1246

Figure 6. Circuit for audio ADCs with balanced inputs

Vout

6

In+

3

In-

2

U1

1243

In Hi

In Lo

Out Hi

Out Lo

In

4

Gnd

3

Out-

1

Out+

8

U2

THAT1430

C1

100n

R1

1M0

R2

20k0

R4

5k1

R3

14k0

C2

22u

C3

22p

IN

1

8

OUT

U3

2180B

3

2

1

U4A

4570

VEE

VCC

VCC

VEE

Control Voltage

-3dB

+6dB

Vee

Cap2

Vcc

Cap1

6

7

2

5

GND

EC-

EC+

SYM

V-

V+

5

6

3

2

4

7

Sense

Ref

5

1

-3dB

Figure 7. Automated gain control of a balanced signal