1293Q16-U THAT Corporation, 1293Q16-U Datasheet - Page 8

• Manufacturer Part Number: 1293Q16-U
• Description: Difference Amplifiers Dual Bal. Line Receiver -3 db
• Manufacturer: THAT Corporation

Specifications of 1293Q16-U

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Document 600121 Rev 01
Driving Analog-to-Digital Converters
a typical audio ADC with balanced inputs.
circuit accepts +24 dBu in. By using both halves of a
single
maximum signal level between their respective
outputs is +27 dBu. An attenuator network brings
Input 2
In+
Input 1
In-
Figure 12 shows a convenient method of driving
In+
In-
100k
100k
Figure 10. Precision two-input summing circuit
R4
R5
1293
2/6
3/5
R1
Rg
Figure 11. Instrumentation amplifier
2/6
3/5
In-
In+
Figure 9. +6 dB line receiver
IC
6
5
3
2
In-
In+
9k
9k
4k99
4k99
R2
R3
U2A
OP-270
U2B
12k
12k
OP-270
connected
7
1
13
4
V
V
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
13
CC
EE
4
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
V
V
CC
EE
9k
9k
100n
2
3
C1
6k
6k
100n
C2
In-
In+
Sense
in
U1
1290
Vout
Sense
V
U1
1296
V
Vout
Ref
V
CC
CC
Ref
V
EE
13
EE
4
Sens
anti-phase,
Ref
14/12
15/11
16/10
14/12
15/11
16/10
16
14
Out
Copyright © 2008, THAT Corporation
THAT1290
U1
15
Output
Output
Out
This
the
Page 8 of 10
this signal down by 18.8 dB while attenuating the
noise of the line receivers as well.
usually a significant consideration. The output noise
of one channel of a THAT 1293 is -105.5 dBu in a
22 kHz bandwidth, or 27.8 nV/√Hz.
channels are used, and since noise adds in random
fashion (square-root of the sum of the squares), the
total noise level at the input of the resistive pad (R
R
reduces this noise level to -121.3 dBu or 4.5 nV/√Hz
at the input to the ADC, while C
filtering typically required by ADCs.
1.87 nV/√Hz or the equivalent noise of a 210 Ω resis-
tor. Therefore, the total noise density going into the
input of the ADC will be
e
Noise
Controlling Gain in Balanced Systems
balanced system, designers are often tempted to keep
the signal balanced and use two Voltage Controlled
Amplifiers (VCAs) to control the gain on each half of
the balanced signal. Unfortunately, this can result in
common-mode
(degrading CMRR) when there are even slight differ-
ences in gain between the VCAs. A better approach is
to convert the signal to single-ended, alter the gain,
and then convert back to balanced.
balanced system. First, we use a 1293 -3 dB line
receiver to perform the balanced to single-ended
conversion. A THAT 1606, with +6dB gain, is used
to rebalance the signal before the circuit’s output. A
THAT 2162 dual VCA is used to alter gain based on a
dc voltage applied at E
(This point is intended to be driven from a
low-impedance, low-noise voltage source.
THAT 2162-series data sheet for details.)
“static” gain of -3 dB (gain with 0 Vdc applied to the
E
Additionally, the 1293 has a gain of -3 dB for a total
attenuation of 6 dB before the output driver. The
1606 has a gain of 6 dB, therefore the circuit has a
gain of 0 dB with 0 V at the control voltage node.
before clipping, and has a noise floor of -91.5 dBu
(22 kHz bandwidth). By varying the Control Voltage,
gains from -70 dB to +40 dB may easily be achieved.
The VCA’s “deci-linear” relationship between Control
Voltage and gain makes the gain setting precise,
predictable, and repeatable.
n ADC input
3
C-
) will be -102.9 dBu or 37.5 nV/√Hz.
) due to the choice of ratio of R
In ADC applications such as this, noise is
The thermal noise of the resistive attenuator is
The noise floor can then be calculated to be
When it becomes necessary to control gain in a
Figure 13 shows a stereo gain control for a
As shown, the VCA section is configured for
This circuit accepts and delivers over +24 dBu
( dBu )
= (1.87
= 20 log
Low Cost Dual Balanced Line Receivers
to
4.87
nV
Hz
C-
differential-mode
, the “Control Voltage” node.
nV
)
Hz
0.775
2
+ (4.5
% 22kHz
1
3
nV
Hz
to R
provides low-pass
= −120.6 dBu
THAT 1290 Series
)
2
2
and R
= 4.87
Since both
conversion
The pad
See the
7
to R
nV
Hz
.
1
~
6
.
.