Datasheet
DIFFERENTIAL I/O APPLICATIONS
R
500W
F
R
500W
F
OPA2695
+V
CC
-V
CC
+V
CC
-V
CC
R
G
V
O
OPA2695
V
I
R
500W
F
R
500W
F
R
G
R
G
OPA2695
+V
CC
-V
CC
V
CM
V
CM
-V
CC
V
O
OPA2695
V
I
OPA2695
SBOS354A – APRIL 2008 – REVISED AUGUST 2008 ......................................................................................................................................................
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frequency response. It is common for ac-coupled
applications to include a blocking capacitor in series
The OPA2695 offers very low third-order distortion
with R
G
. This reduces the gain to 1 at low frequency,
terms with a dominant second-order distortion for the
rising to the A
D
expression shown above at higher
single amplifier operation. For the lowest distortion,
frequencies. The noninverting input approach of
particularly where differential outputs are needed,
Figure 74 can be used for higher gains than the
operating two OPA2695s in a differential I/O design
inverting input approach. It does, however, have a
suppresses these even-order terms, delivering
reduced full-power bandwidth because of the lower
extremely low harmonic distortion through high
slew rate of the OPA2695 running noninverting
frequencies and powers. Differential outputs are often
versus inverting input mode of operation.
preferred for high performance ADCs, twisted-pair
driving, and mixer interfaces. Two basic approaches Various combinations of single-supply or ac-coupled
to differential I/Os are the noninverting or inverting gain can also be delivered using the basic circuit of
configurations. Since the output is differential, the Figure 74 . Common-mode bias voltages on the two
signal polarity is somewhat meaningless — the noninverting inputs pass on to the output with a gain
noninverting and inverting terminology applies here to of 1, since an equal dc voltage at each inverting node
where the input is brought into the two OPA2695s. creates no current through R
G
. This circuit does show
Each approach has its advantages and a common-mode gain of 1 from input to output. The
disadvantages. Figure 74 shows a basic starting point source connection should either remove this
for noninverting differential I/O applications. common-mode signal if undesired (using an input
transformer can provide this function), or the
common-mode voltage at the inputs can be used to
set the output commonmode 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 transformer.
Figure 75 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.
Figure 74. Noninverting Input Differential I/O
Amplifier
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 noninverting inputs
without interacting with the gain setting. The
differential signal gain for the circuit of Figure 74 is:
A
D
= 1 + 2 × R
F
/R
G
Since the OPA2695 is a current-feedback amplifier,
its bandwidth is principally controlled with the
feedback resistor value — Figure 74 shows a typical
value of 500 Ω . However, the differential gain 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 Figure 75. Inverting Input Differential I/O Amplifier
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