Datasheet

OPA695
22
SBOS293G
www.ti.com
DIFFERENTIAL I/O
APPLICATIONS
The OPA695 offers very low 3rd-order distortion terms with
a dominant 2nd-order distortion for the single amplifier op-
eration. For the lowest distortion, particularly where differen-
tial outputs are needed, operating two OPA695s in a differ-
ential I/O design will suppress these even-order terms, deliv-
ering extremely low harmonic distortion through high fre-
quencies and powers. Differential outputs are often preferred
for high performance ADCs, twisted-pair driving, and mixer
interfaces. Two basic approaches to differential I/Os are the
noninverting or inverting configurations. Since the output is
differential, the signal polarity is somewhat meaningless—
the noninverting and inverting terminology applies here to
where the input is brought into the two OPA695s. Each
approach has its advantages and disadvantages. Figure 13
shows a basic starting point for non-inverting differential I/O
applications.
applications to include a blocking capacitor in series with R
G
.
This reduces the gain to 1 at low frequency, rising to the A
D
expression shown above at higher frequencies. The
noninverting input approach of Figure 13 can be used for
higher gains than the inverting input approach. It will, how-
ever, have a reduced full-power bandwidth due to the lower
slew rate of the OPA695 running noninverting vs inverting
input mode of operation.
Various combinations of single-supply or AC-coupled gain
can also be delivered using the basic circuit of Figure 13.
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 a problem, 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 remove
the common-mode signal at the secondary of the trans-
former.
Figure 14 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 non-inverting
configuration, but does limit the flexibility in setting the input
impedance separately from the gain.
R
F
500Ω
R
F
500Ω
OPA695
+V
CC
–V
CC
+V
CC
–V
CC
R
G
V
O
OPA695
V
I
FIGURE 13. Noninverting Input Differential I/O Amplifier.
FIGURE 14. Inverting Input Differential I/O Amplifier.
R
F
500Ω
R
F
500Ω
R
G
R
G
OPA695
+V
CC
–V
CC
V
CM
V
CM
–V
CC
V
O
OPA695
V
I
This approach allows 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 non-inverting inputs without interacting with the
gain setting. The differential signal gain for the circuit of
Figure 13 is:
A
D
= 1 + 2 • R
F
/R
G
Since the OPA695 is a current feedback amplifier, its band-
width is principally controlled with the feedback resistor
value—Figure 13 shows a typical value of 500Ω. However,
the differential gain may be adjusted with considerable free-
dom using just the R
G
resistor. In fact, R
G
may be a reactive
network providing a very isolated shaping to the differen-
tial frequency response. It is common for AC-coupled
(6)
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