OPA3682 Datasheet, PDF (13/19 Pages) Burr-Brown (TI) – Triple, Wideband, Fixed Gain BUFFER AMPLIFIER With Disable

APPLICATIONS INFORMATION
WIDEBAND BUFFER OPERATION
The OPA3682 gives the exceptional AC performance of a
wideband, current-feedback op amp with a highly linear,
high power output stage. It features internal R_F and R_G
resistors which make it easy to select a gain of +2, +1 or
–1 without external resistors. Requiring only 6mA/ch quies-
cent current, the OPA3682 will swing to within 1V of either
supply rail and deliver in excess of 135mA guaranteed at
room temperature. This low output headroom requirement,
along with supply voltage independent biasing, gives re-
markable single (+5V) supply operation. The OPA3682 will
deliver greater than 200MHz bandwidth driving a 2Vp-p
output into 100Ω on a single +5V supply. Previous boosted
output stage amplifiers have typically suffered from very
poor crossover distortion as the output current goes through
zero. The OPA3682 achieves a comparable power gain with
much better linearity.
Figure 1 shows the DC-coupled, gain of +2, dual power
supply circuit configuration used as the basis of the ±5V
Specifications and Typical Performance Curves. For test
purposes, the input impedance is set to 50Ω with a resistor
to ground and the output impedance is set to 50Ω with a
series output resistor. Voltage swings reported in the speci-
fications are taken directly at the input and output pins
while load powers (dBm) are defined at a matched 50Ω
load. For the circuit of Figure 1, the total effective load will
be 100Ω || 800Ω = 89Ω. The disable control line (DIS) is
typically left open to guarantee normal amplifier operation.
In addition to the usual power supply decoupling capacitors
to ground, a 0.1µF capacitor can be included between the
two power supply pins. This optional capacitor will typi-
cally improve the 2nd harmonic distortion performance by
3dB to 6dB.
50Ω Source
V_IN
50Ω
+5V
DIS
0.1µF
6.8µF
1/3
OPA3682
50Ω Load
50Ω
Figure 2 shows the AC-coupled, gain of +2, single-supply
circuit configuration used as the basis of the +5V Specifica-
tions and Typical Performance Curves. Though not a “rail-
to-rail” design, the OPA3682 requires minimal input and
output voltage headroom compared to other very wideband,
current-feedback op amps. It will deliver a 3Vp-p output
swing on a single +5V supply with greater than 150MHz
bandwidth. The key requirement of broadband single-supply
operation is to maintain input and output signal swings
within the usable voltage ranges at both the input and the
output. The circuit in Figure 2 establishes an input midpoint
bias using a simple resistive divider from the +5V supply
(two 806Ω resistors). The input signal is then AC-coupled
into this midpoint voltage bias. The input voltage can swing
to within 1.5V of either supply pin, giving a 2Vp-p input
signal range centered between the supply pins. The input
impedance matching resistor (57.6Ω) used for testing is
adjusted to give a 50Ω input match when the parallel
combination of the biasing divider network is included. The
gain resistor (R_G) is AC-coupled, giving the circuit a DC
gain of +1, which puts the input DC bias voltage (2.5V) on
the output as well. Again, on a single +5V supply, the output
voltage can swing to within 1V of either supply pin while
delivering more than 75mA output current. A demanding
100Ω load to a midpoint bias is used in this characterization
circuit. The new output stage used in the OPA3682 can
deliver large bipolar output currents into this midpoint load
with minimal crossover distortion, as shown by the +5V
supply, 3rd harmonic distortion plots.
+V_S +5V
50Ω Source
0.1µF
V_IN
57.6Ω
806Ω
806Ω
0.1µF
6.8µF
1/3
OPA3682
DIS
V_O 100Ω
R_F
400Ω
V_S/2
R_G
400Ω
0.1µF
R_F
400Ω
R_G
400Ω
0.1µF
6.8µF
–5V
FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifi-
cation and Test Circuit.
FIGURE 2. AC-Coupled, G = +2, Single-Supply Specifica-
tion and Test Circuit.
VIDEO RGB AMPLIFIER
The front page shows an RGB amplifier based on the
OPA3682. The package pinout supports a signal flow-through
PCB layout. The internal resistors simplify the PCB even
more, while maintaining good gain accuracy. For systems that
need to conserve power, the total supply current for the
disabled OPA3682 is only 0.9mA.
®
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OPA3682