Application Notes

The THAT 1240, 1243, and 1246 are precision

differential amplifiers with gains of zero, -3 and -6 dB

respectively, and are primarily intended as balanced

line receivers for pro audio. However, their topology

lends itself to other applications as well.

Figures 3 and 4 show the THAT 1240 and

THAT 1246 configured as zero and -6 dB line receiv-

ers respectively. These devices can be used to retrofit

circuits currently using the SSM 2141, INA134,

SSM2143, or INA137.

While the THAT 1240 and

THAT 1246 are form, fit and functionally compatible

with their competitors’ equivalent versions, their slew

rate and bandwidth are superior.

Like other amplifiers of this type, all of THAT

Corporation’s line receivers can accept common

mode voltages that exceed the power supply rails.

Consider Figure

2. Assume that the circuit is con-

figured as shown, but with no differential excitation.

In+ and In- are shorted together, as are Vout and

Sense. Ref is tied to ground. a and b will be held at

the same voltage by feedback.

The maximum voltage allowed at the internal

V

The voltage at a can be calculated

=

-

+

[

]

R4

R3 R4

Isolating V

Va

PK CM

RR

R

-

+

=

[]

43

4

Thus, the maximum input in dBu would be

dbu

Vcc

V

RR

R

max

.

log

()

[]

-

´

=

é

ë

ê

ê

ù

û

ú

ú

-

+

20

2

2

43

4

0775

With ±18V supplies, the THAT 1240 can tolerate

a maximum common mode input of 29.3 dBu. Un-

der the same conditions, the THAT1243 can tolerate

30.0 dBu and the THAT 1246 can tolerate 32.9 dBu.

These numbers are slightly higher than the datasheet

specifications, since they’re calculated with a slightly

higher supply voltage. Also note that high levels of

common mode input compromise the maximum dif-

ferential mode signal that can be handled by these

ICs.

Figure 5 shows a THAT 1240 configured as a pre-

cision summing amplifier. This circuit uses both the

In+ and Ref pins as inputs. Refering to Figure 2, it

can be shown by superposition that the voltage at a

will be

aIn

R

R

RR

R

RR

=+

+

++

(

) [][]

4

43

3

43

ef

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

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

Page 4

Balanced Line Receiver ICs

Preliminary Information

Vout

6

In+

3

In-

2

U1

1240

Hi

Lo

Hi - Lo

Sense

Ref

1

5

Figure 3. Zero dB line receiver

Vout

6

In+

3

In-

2

U1

1246

Hi

Lo

(Hi-Lo )*0.5

Sense

Ref

1

5

Figure 4. -6 dB line receiver

Vout

6

In+

3

In-

2

U1

1240

In+

Ref

(In+)+ Ref

Sense

Ref

1

5

Figure 5. Precision summing circuit