Datasheet

OPA2694
SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013
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14
common-mode signals very well, while a line driver
application through a transformer also attenuates the
common-mode signal through to the line.
Figure 7 shows a differential I/O stage configured as an
inverting amplifier. In this case, the gain resistors (R
G
)
become part of the input resistance for the source. This
provides a better noise performance than the noninverting
configuration, but does limit the flexibility in setting the
input impedance separately from the gain.
R
F
R
F
R
G
R
G
1/2
OPA2694
1/2
OPA2694
+V
CC
−
V
CC
V
CM
V
CM
V
O
V
I
Figure 7. Inverting Differential I/O Amplifier
The two noninverting inputs provide an easy
common-mode control input. This is particularly easy if the
source is AC-coupled through either blocking caps or a
transformer. In either case, the common-mode input
voltages on the two noninverting inputs again have a gain
of 1 to the output pins, giving particularly easy
common-mode control for single-supply operation. Once
R
F
is fixed, the input resistors can be adjusted to the
desired gain, but will also be changing the input
impedance as well. The high-frequency, common-mode
gain for this circuit from input to output is the same as for
the signal gain. Again, if the source includes an undesired
common-mode signal, it can be rejected at the input using
blocking caps (for low-frequency and DC common-mode)
or a transformer coupling.
DC-COUPLED SINGLE-TO-DIFFERENTIAL
CONVERSION
The previous differential output circuits were set up to
receive a differential input as well. A simple way to provide
a DC-coupled single-to-differential conversion using a
dual op amp is shown in Figure 8. Here, the output of the
first stage is simply inverted by the second to provide an
inverting version of a single amplifier design. This
approach works well for lower frequencies, but will start to
depart from ideal differential outputs as the propagation
delay and distortion of the inverting stage adds
significantly to that present at the noninverting output pin.
402
Ω
12V
PP
Differential
402
Ω
1/2
OPA2694
+5V
−
5V
402
Ω
80.6
Ω
1/2
OPA2694
50
Ω
1V
PP
Figure 8. Single-to-Differential Conversion
The circuit of Figure 8 is set up for a single-ended gain of
6 to the output of the first amplifier, then an inverting gain
of –1 through the second stage to provide a total
differential gain of 12.