THAT1240 Datasheet, PDF (4/8 Pages) List of Unclassifed Manufacturers – Balanced Line Receiver ICs

Page 4
Balanced Line Receiver ICs
Preliminary Information
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.
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
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.
a
=
(In+)
[
R
R4
4+R
3^]
+
Ref
[
R
R3
4+R
3^]
Lo ₂
In-
5
Sense
6
Vout
Hi ₃
Ref
In+ ₁
U1
1240
Hi - Lo
Figure 3. Zero dB line receiver
The maximum voltage allowed at the internal
node a is V_CC minus 2V, while the minimum voltage is
V_EE plus 2V. The voltage at a can be calculated
] a
=
V_{PK -CM}
[
R4
R3+ R4
Isolating V_PK-CM, we see that
V_{PK -CM}
=
a
[
R
4+R
R4
3
]
Thus, the maximum input in dBu would be
V_{IN max -dbu}
=
20 log
é ^{(Vcc -2V)}
ê²
´
[^R
4+ R
R4
3_]ù
ú
ëê
0.775
ûú
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.
Lo _{2 In- 5}
Sense
6 ^{( Hi - Lo ) * 0.5}
Vout
Hi _{3 Ref}
In+ _{1 U1}
1246
Figure 4. -6 dB line receiver
2
In-
5
Sense
6 ^{(In+)+ Ref}
Vout
In+ _{3 Ref}
In+ ₁
Ref
U1
1240
Figure 5. Precision summing circuit
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com