OPA2682 Datasheet, PDF (13/19 Pages) Burr-Brown (TI) – Dual, Wideband, Fixed Gain BUFFER AMPLIFIER With Disable

APPLICATIONS INFORMATION
WIDEBAND BUFFER OPERATION
The OPA2682 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 any external resistors. Requiring only 6mA/ch
quiescent current, the OPA2682 will swing to within 1V of
either supply rail and deliver in excess of 135mA (guaran-
teed) at room temperature. This low output headroom re-
quirement, along with supply voltage independent biasing,
gives remarkable single (+5V) supply operation. The
OPA2682 will deliver greater than 200MHz bandwidth driv-
ing a 2Vp-p output into 100Ω on a single +5V supply.
Previous current-boosted output stage amplifiers have typi-
cally suffered from very poor crossover distortion as the
output current goes through zero. The OPA2682 achieves a
comparable power gain with much better linearity. The
primary advantage of the internal current-feedback op amp
over a voltage-feedback op amp is that AC performance
(bandwidth and distortion) is relatively independent of sig-
nal gain.
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 (SO-14 only) to guarantee normal ampli-
fier 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
added capacitor will typically improve the 2nd harmonic
distortion performance by 3dB to 6dB.
+5V
+V_S
50Ω Source
0.1µF
6.8µF
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 OPA2682 requires minimal input and
output voltage headroom compared to other fixed-gain buffer
amplifiers. 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 main-
tain input and output signal swings within the usable voltage
ranges at both the input and the output. The circuit of 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. 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 demand-
ing 100Ω load to a midpoint bias is used in this characteriza-
tion circuit. The new output stage used in the OPA2682 can
deliver large bipolar output currents into this midpoint load
with minimal crossover distortion, as shown by the +5V
supply, 3rd harmonic distortion plots.
+5V
+V_S
50Ω Source
0.1µF
V_IN
57.6Ω
806Ω
806Ω
0.1µF
6.8µF
1/2
OPA2682
V_O 100Ω
R_F
400Ω
V_S/2
R_G
400Ω
0.1µF
V_IN
50Ω
1/2
OPA2682
50Ω Load
50Ω
R_F
400Ω
R_G
400Ω
–5V
–V_S
0.1µF
6.8µF
FIGURE 2. AC-Coupled, G = +2, Single Supply Specifica-
tion and Test Circuit.
HIGH SPEED INSTRUMENTATION AMPLIFIER
The front page shows an instrumentation amplifier based on
the OPA2682. The differential gain for this circuit is 2.0. The
inputs are high impedance, with only 1pF to ground at each
input. The loads on the OPA2682 outputs were made equal in
order to achieve the best harmonic distortion possible.
FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifi-
cation and Test Circuit.
13
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OPA2682