Datasheet
ADA4075-2 Data Sheet
Rev. C | Page 18 of 24
BALANCED LINE DRIVER
The circuit of Figure 67 shows a balanced line driver designed
for audio use. Such drivers are intended to mimic an output
transformer in operation, whereby the common-mode voltage
can be impressed by the load. Furthermore, either output can be
shorted to ground in single-ended applications without affecting
the overall operation.
Circuits of this type use positive and negative feedback to obtain a
high common-mode output impedance, and they are somewhat
notorious for component sensitivity and susceptibility to latch-up.
This circuit uses several techniques to avoid spurious behavior.
First, the 4-op-amp arrangement ensures that the input impedance
is load independent (the input impedance can become negative
with some configurations). Note that the output op amps are
packaged with the input op amps to maximize drive capability.
Second, the positive feedback is ac-coupled by C2 and C3, which
eliminates the need for offset trim. Because the circuit is ac-coupled
at the input, these capacitors do not have significant dc voltage
across them, thus tantalum types of capacitors can be used.
Finally, even with these precautions, it is vital that the positive
feedback be accurately controlled. This is partly achieved by
using 1% resistors. In addition, the following setup procedure
ensures that the positive feedback does not become excessive:
1. Set R11 to its midposition (or short the ends together,
whichever is easier) and temporarily short the negative
output to ground.
2. Apply a 10 V p-p sine wave at approximately 1 kHz to the
input and adjust R7 to provide 930 mV p-p at TEST (see
Figure 67).
3. Remove the short from the negative output (and across
R11, if used) and adjust R11 until the output waveforms
are symmetric.
The overall gain of the driver is equal to 2, which provides an
extra 6 dB of headroom in balanced differential mode. The
output noise is about −109 dBV in a 20 kHz bandwidth.
07642-073
C3
10
µF
R2
4.7kΩ
OUT+
OUT–
1/2
ADA4075-2
R5
4.7kΩ
R6
4.7k
Ω
R7
250Ω
R8
100Ω
R9
4.7kΩ
R13
100Ω
R4
4.7kΩ
A2
1/2
ADA4075-2
A1
IN
C5
50pF
C1
10
µF
C2
10
µF
TEST
SYMMETRY
TRIM
FEEDBACK
TRIM
1/2
ADA4075-2
R12
4.7kΩ
R11
250Ω
R17
4.7kΩ
R16
100
Ω
R10
4.7kΩ
R15
4.7kΩ
A4
C6
50pF
1/2
ADA4075-2
A3
C4
50pF
R14
100Ω
R1
10k
Ω
NOTES
1. ALL RESIS
TORS SHOULD HAVE 1% TOLERANCE.
2. A1/A2 IN SAME PACKAGE; A3/A4 IN SAME PACKAGE.
R3
4.7k
Ω
R18
4.7kΩ
Figure 67. Balanced Line Driver