that1250 THAT Corporation, that1250 Datasheet - Page 6

• Manufacturer Part Number: that1250
• Description: Low Cost, Balanced Line Receiver Ics
• Manufacturer: THAT Corporation

Document 600068 Rev 02
thought of as precision differential amplifiers with
gains of zero, -3 and -6 dB respectively. These
devices are primarily intended as balanced line
receivers for audio applications.
topology lends itself to other applications as well.
Basic Balanced Receiver Applications
1250, 1256, and 1253 configured as zero, -6 dB, and
-3 dB line receivers. Figures 8 and 9, respectively,
show the 1253 and 1256 configured as +3 dB and
+6 dB line receivers. The higher gains are achieved
by swapping the positions of the resistors within
each pair in regard to signal input vs. Output. In all
five cases, no external resistors are required to set
the desired gain.
precision summing amplifier. This circuit uses both
the In+ and Ref pins as inputs. Because of the good
matching between the resistor pairs, the output
voltage is precisely equal to the sum of the two input
voltages.
More Complex Applications
a typical audio ADC with balanced inputs.
circuit accepts +24 dBu in. By using a pair of THAT
1256 ICs connected in anti-phase, the signal level
between their respective outputs is +24 dBu.
attenuator network brings this signal down by 24 dB
The THAT 1250, 1253, and 1256 are usually
Figures 5, 6, and 7, respectively, show the THAT
Figure 10 shows a THAT 1250 configured as a
Figure 11 shows a convenient method of driving
-In
+In
+In
-In
2
3
2
3
In+
In+
In-
In-
Figure 6. -3 dB line receiver
Figure 7. -6 dB line receiver
10.5k
10.5k
12k
12k
4
4
7
7
V
V
V
V
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
CC
EE
CC
EE
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
7.5k
7.5k
U1
THAT1256
U1
THAT1253
6k
6k
Sense
Sense
Vout
Vout
Ref
Ref
However, their
5
6
1
5
6
1
Copyright © 2008, THAT Corporation
Output
Output
Applications
This
An
Page 6 of 8
while attenuating the noise of the line receivers as
well.
Since there are two of them, and uncorrelated noise
sources add in RMS fashion, the total noise level
going into the resistive pad will be 3 dB higher, or
-103 dBu. The pad reduces the noise level to
-127 dBu at the input to the ADC. The noise density
resulting from the line receivers will therefore be
2.05 nV/√Hz. We can assume that the noise contri-
bution of R
the total noise density going into the input of the ADC
will be
Noise
controlling gain in a balanced system. In such
circuits, designers are sometimes tempted to keep
the signal balanced and use two Voltage Controlled
Amplifiers (VCAs) to independently control the gain
on each half of the balanced signal. Unfortunately,
this can result in common-mode to differential-mode
The output noise of a THAT 1256 is -106 dBu.
e
The thermal noise of the 249 Ω resistor is
e
The noise floor can then be calculated to be
Figure 12 shows the recommended method for
n total
n line receiver
( dBu )
+In
-In
+In
-In
= (2.45
= 20 log
8
and R
Figure 9. +6 dB line receiver
Figure 8. +3 dB line receiver
2
3
2
3
=
In+
In+
In-
In-
19
Low-cost Balanced Line Receiver ICs
10
10.5k
10.5k
12k
12k
will be negligible, and therefore,
nV
3.2
Hz
−127dBu
20
)
20kHz
nV
Hz
4
4
7
7
2
V
V
V
V
0.775
CC
EE
CC
EE
+ (2.06
% 20kHz
%0.775
U1
THAT1253
7.5k
7.5k
U1
THAT1256
6k
6k
Sense
Sense
Vout
Vout
Ref
Ref
= 2.45
nV
THAT 1250 Series
Hz
= −124.7 dBu
5
6
5
6
1
1
)
2
Output
= 3.2
Out
nV
Hz
nV
Hz