OPA2684IDCNT Datasheet, PDF (16/34 Pages) Texas Instruments – Low-Power, Dual Current-Feedback OPERATIONAL AMPLIFIER

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:
A_D = 1 + 2 • R_F /R_G
Since the OPA2684 is a CFB_PLUS amplifier, its bandwidth is
principally controlled with the feedback resistor value, Figure 5
shows the recommended value of 800Ω. The differential
gain, however, may be adjusted with considerable freedom
using just the R_G resistor. In fact, R_G may be a reactive
network providing a very isolated shaping to the differential
frequency response. Since the inverting inputs of the OPA2684
are very low impedance closed-loop buffer outputs, the R_G
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.
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
R_G. 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 problem, the output interface may also be used to reject
that common-mode. For instance, most modern differential
input ADC’s 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.
Figure 6 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.
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. The OPA2684 used in this configuration
does constrain the feedback to the 800Ω region for best
frequency response. With R_F 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 could 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 re-
ceive 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 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 inverting stage adds significantly to that present at the
noninverting output pin.
1Vp-p
+5V
50Ω
1/2
OPA2684
800Ω
160Ω
800Ω
12Vp-p Differential
V_CM
R_G
V_I
R_G
+V_CC
1/2
OPA2684
R_F
800Ω
R_F
800Ω
800Ω
1/2
OPA2684
–5V
V_O
FIGURE 7. Single to Differential Conversion.
1/2
OPA2684
V_CM
–V_CC
FIGURE 6. Inverting Differential I/O Amplifier.
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 SSBW for the circuit of Figure 7.
Large-signal distortion at 12Vp-p output into the 100Ω differ-
ential load is ≤ 80dBc.
16
OPA2684
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