OPA2681 Datasheet, PDF (14/21 Pages) Burr-Brown (TI) – Dual Wideband, Current Feedback OPERATIONAL AMPLIFIER With Disable

stability. This first stage easily drives the low input resistors
required at the input of this high frequency filter. The 2nd
stage is set for a DC gain of +1—carrying the 2.5V operating
point through to the output pin, and an AC gain of +4. The
feedback resistor has been adjusted to optimize bandwidth
for the amplifier itself. As the single-supply frequency
response plots show, the OPA2681 in this configuration will
give > 200MHz small signal bandwidth. The capacitor
values were chosen as low as possible but adequate to
swamp out the parasitic input capacitance of the amplifier.
The resistor values were slightly adjusted to give the desired
filter frequency response while accounting for the approxi-
mate 1ns propogation delay through each channel of the
OPA2681.
HIGH POWER TWISTED PAIR DRIVER
A very demanding application for a high-speed amplifier is to
drive a low load impedance while maintaining a high output
voltage swing to high frequencies. Using the dual current
feedback op amp OPA2681, a 15Vp-p output signal swing
into a twisted-pair line with a typical impedance of 100Ω can
be realized. Configured as shown in the front page the two
amplifiers of the OPA2681 drive the output transformer in a
push-pull configuration thus doubling the peak-to-peak signal
swing at each op amp’s output to 15Vp-p. The transformer has
a turns ratio of 2. In order to provide a matched source, this
requires a 25Ω source impedance (R_S), for the primary side,
given the transformer equation n² = R_L/R_S. Dividing this
impedance equally between the outputs requires a series
termination matching resistor at each output of 12.4Ω. Taking
the total resistive load of 25Ω (for the differential output
signal) and drawing a load line on the Output Voltage and
Current Limitations plot it can be seen a 1.5V headroom is
required at the positive peak current of 150mA, while a 2.5V
headroom is required at the negative peak current of 150mA.
The full 7.5Vp-p out of each amplifier is achieved on a single
+12V supply by shifting the DC operating point positive 0.5V
to 6.5V—as shown on the front page ADSL upstream driver.
Line driver applications usually have a high demand for
transmitting the signal with low distortion. Current-feedback
amplifiers like the OPA2681 are ideal for delivering low
distortion performance to higher gains. The example shown is
set for a differential gain of 7.5. This circuit can deliver the
maximum 15Vp-p signal with over 60 MHz bandwidth.
WIDEBAND (160MHZ) INSTRUMENTATION AMPLIFIER
As discussed previously, the current feedback topology of
the OPA2681 provides a nearly constant bandwidth as signal
gain is increased. The three op amp wideband instrumenta-
tion amplifier depicted in Figure 6 takes advantage of this,
achieving a differential bandwidth of 160MHz. The signal is
applied to the high-impedance non-inverting inputs of the
OPA2681. The differential gain is set by (1 + 2R_F/ R_G) —
which equal to 5 using the values shown in Figure 6. The
feedback resistors, R_F, are optimized at this particular gain.
Gain adjustments can be made by adjusting R_G. The differ-
ential to single-ended conversion is performed by voltage
feedback amplifier OPA680 configured as a standard differ-
ence amplifier. To maintain good distortion performance for
the OPA2681, the loading at each amplifier output has been
matched by setting R3 + R4 = R1, rather than using the same
resistor values within the difference amplifier.
+5V
V_IN
1/2
OPA2681
R_F
261Ω
R_G
130Ω
R_F
261Ω
1/2
OPA2681
V_IN
–5V
R₁
499Ω
R₂
499Ω
+5V
OPA680
V_O
R₃
249Ω
–5V
R₄
249Ω
V_O
=5
(V_IN – V_IN)
FIGURE 6. Wideband, 3-Op Amp Instrumentation Diff. Amp.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Several PC boards are available to assist in the initial
evaluation of circuit performance using the OPA2681 in its
2 package styles. Both of these are available free as an
unpopulated PC board delivered with descriptive documen-
tation. The summary information for these boards is shown
in the table below.
PRODUCT
OPA2681U
OPA2681N
PACKAGE
8-Pin SO-8
14-Pin SO-14
DEMO BOARD
NUMBER
DEM-OPA268xU
DEM-OPA268xN
ORDERING
NUMBER
MKT-352
MKT-353
Contact the Burr-Brown applications support line to request
any of these boards.
MACROMODELS AND APPLICATIONS SUPPORT
Computer simulation of circuit performance using SPICE is
often useful when analyzing the performance of analog
circuits and systems. This is particularly true for Video and
RF amplifier circuits where parasitic capacitance and induc-
tance can have a major effect on circuit performance. A
SPICE model for the OPA2681 is available through either
the Burr-Brown Internet web page (http://www.burr-
brown.com) or as one model on a disk from the Burr-Brown
Applications department (1-800-548-6132). The Applica-
tion department is also available for design assistance at this
number. These models do a good job of predicting small
signal AC and transient performance under a wide variety of
®
OPA2681
14