OPA2691 Datasheet, PDF (14/30 Pages) Texas Instruments – Dual Wideband, Current-Feedback OPERATIONAL AMPLIFIER

The 51Ω input matching resistor is optional in this case. The
input signal is AC-coupled to the 2.5V DC reference voltage
developed through the resistor divider from the +5V power
supply. This first stage acts as a gain of +1 voltage buffer for
the signal where the 600Ω feedback resistor is required for
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 re-
sponse plots show, the OPA2691 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 fre-
quency response while accounting for the approximate 1ns
propagation delay through each channel of the OPA2691.
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 OPA2691, a 15Vp-p output signal swing
into a twisted-pair line with a typical impedance of 100Ω can
be realized. Configured as shown on the front page, the two
amplifiers of the OPA2691 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 that a 1.5V headroom
is required at both the positive and negative peak currents of
150mA.
Line driver applications usually have a high demand for
transmitting the signal with low distortion. Current-feedback
amplifiers like the OPA2691 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 60MHz bandwidth.
WIDEBAND (200MHz) INSTRUMENTATION AMPLIFIER
As discussed previously, the current-feedback topology of
the OPA2691 provides a nearly constant bandwidth as signal
gain is increased. The three op amp wideband instrumenta-
tion amplifiers depicted in Figure 6 takes advantage of this,
achieving a differential bandwidth of 200MHz. The signal is
applied to the high-impedance noninverting inputs of the
OPA2691. The differential gain is set by (1 + 2R_F/R_G)—which
is 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 the volt-
age-feedback amplifier OPA690, configured as a standard
difference amplifier. To maintain good distortion performance
for the OPA2691, 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
OPA2691
R_F
261Ω
R_G
130Ω
R_F
261Ω
1/2
OPA2691
V_IN
–5V
R₁
499Ω
R₂
499Ω
+5V
OPA690
V_O
R₃
249Ω
–5V
R₄
249Ω
V_O
=5
(V_IN – V_IN)
INSTRUMENTATION DIFF. AMP
FREQUENCY RESPONSE
15
10
5
0
1
10
100
400
Frequency (MHz)
FIGURE 6. Wideband, 3-Op Amp Instrumentation Diff. Amp.
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist in
the initial evaluation of circuit performance using the OPA2691
in its two package options. Both of these are offered free of
charge as unpopulated PCBs, delivered with a user’s guide.
The summary information for these fixtures is shown Table I
below.
PRODUCT
PACKAGE
ORDERING
NUMBER
LITERATURE
NUMBER
OPA2691ID
OPA2691I-14D
SO-8
SO-14
DEM-OPA-SO-2A
DEM-OPA-SO-2B
SBOU003
SBOU002
TABLE I. Demonstration Fixtures by Package.
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA2691
product folder.
14
OPA2691
www.ti.com
SBOS224D