Datasheet

OPA2677
15
SBOS126I
www.ti.com
The last configuration used as the basis of the +5V Electrical
and Typical Characteristics is shown in Figure 4. Design
considerations for this inverting, bipolar supply configuration
are covered either in single-supply configuration (as shown
in Figure 3) or in the
Inverting Amplifier Operation section.
where the input is brought into the OPA2677. Each has its
advantages and disadvantages. Figure 5 shows a basic
starting point for noninverting differential I/O applications.
DIFFERENTIAL INTERFACE APPLICATIONS
Dual op amps are particularly suitable to differential input to
differential output applications. Typically, these fall into either
Analog-to-Digital Converter (ADC) input interface or line
driver applications. Two basic approaches to differential I/O
are noninverting or inverting configurations. Since the output
is differential, the signal polarity is somewhat meaningless—
the noninverting and inverting terminology applies here to
1/2
OPA2677
+5V
V
S
/2
806Ω
V
I
100ΩV
O
806Ω
R
F
453Ω
R
M
88.7Ω
6.8µF
+
0.1µF
0.1µF
R
G
113Ω
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 OPA2677 is a current feedback (CFB) amplifier, its
bandwidth is principally controlled with the feedback resistor
value; Figure 5 shows a value of 300Ω for the A
D
= +9
design. 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.
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 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 attenu-
ate the common-mode signal through to the line.
R
L
R
F
300Ω
R
F
300Ω
–6
+6
1/2
OPA2677
1/2
OPA2677
R
G
75Ω
C
G
V
O
V
I
G
D
= 1 + =
2 • R
F
R
G
V
O
V
I
FIGURE 3. AC-Coupled, G = +4, Single-Supply, Specifica-
tion and Test Circuit.
FIGURE 4. AC-Coupled, G = –4, Single-Supply, Specifica-
tion and Test Circuit.
FIGURE 5. Noninverting Differential I/O Amplifier.
1/2
OPA2677
+5V
+V
S
V
S
/2
806Ω
100ΩV
O
V
I
57.6Ω
806Ω
R
F
453Ω
R
G
150Ω
0.1µF
0.1µF
6.8µF
+
0.1µF