This approach provides for a source termination impedance
that is independent of the signal gain. For instance, simple
differential filters may be included in the signal path right up
to the noninverting inputs without interacting with the gain
setting. The differential signal gain for the circuit of Figure 5 is:
The two noninverting inputs provide an easy common-mode
control input. This is particularly simple 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. The OPA2683 used in this configuration
does constrain the feedback to the 953Ω region for best
frequency response. With RF fixed, the input resistors may 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 will be the same as
for the signal gain. Again, if the source might include an
undesired common-mode signal, that signal could be re-
jected at the input using blocking caps (for low frequency and
DC common-mode) or a transformer coupling.
(1)
AD = 1 + 2 • RF /RG
Since the OPA2683 is a CFBPLUS amplifier, its bandwidth is
principally controlled with the feedback resistor value; see
Figure 5 for the recommended value of 953Ω. The differential
gain, however, may be adjusted with considerable freedom
using just the RG resistor. In fact, RG may be a reactive
network providing a very isolated shaping to the differential
frequency response. Since the inverting inputs of the OPA2683
are very low impedance closed-loop buffer outputs, the RG
element does not interact with the amplifier’s bandwidth;
wide ranges of resistor values and/or filter elements may be
inserted here with minimal amplifier bandwidth interaction.
DC-COUPLED SINGLE TO DIFFERENTIAL CONVERSION
Various combinations of single-supply or AC-coupled gain
can also be delivered using the basic circuit of Figure 5.
Common-mode bias voltages on the two noninverting inputs
pass on to the output with a gain of 1 since an equal DC
voltage at each inverting node creates no current through
RG. This circuit does show a common-mode gain of 1 from
input to output. The source connection should either remove
this common-mode signal if undesired (using an input trans-
former can provide this function), or the common-mode
voltage at the inputs can be used to set the output common-
mode bias. If the low common-mode rejection of this circuit
is a concern, the output interface may also be used to reject
that common-mode. For instance, most modern differential
input ADCs reject common-mode signals very well, while a
line driver application through a transformer will also attenu-
ate the common-mode signal through to the line.
The previous differential output circuits were also set up to
receive a differential input. A simple way to provide a DC-
coupled single to differential conversion using a dual op amp
is shown in Figure 7. 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 invert-
ing stage adds significantly to that present at the noninverting
output pin.
+5V
1VPP
1/2
50Ω
OPA2683
Figure 6 shows a differential I/O stage configured as an
inverting amplifier. In this case, the gain resistors (RG)
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.
953Ω
191Ω
12VPP Differential
953Ω
953Ω
+VCC
VCM
1/2
OPA2683
1/2
OPA2683
RF
RG
RG
953Ω
–5V
RF
953Ω
FIGURE 7. Single to Differential Conversion.
VI
VO
The circuit of Figure 7 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. See Figure 8 for the 75MHz small-signal band-
width delivered by the circuit of Figure 7. Large-signal distor-
tion at 12VPP output at 1MHz into the 1kΩ differential load is
≤ –76dBc.
1/2
OPA2683
VCM
–VCC
FIGURE 6. Inverting Differential I/O Amplifier.
OPA2683
SBOS244H
15
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