OPA3695_14 Datasheet, PDF (13/39 Pages) Texas Instruments – Triple, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3695
www.ti.com ............................................................................................................................................... SBOS355A – APRIL 2008 – REVISED SEPTEMBER 2008
APPLICATION INFORMATION
WIDEBAND BUFFER OPERATION
The OPA3695 gives the exceptional ac performance
of a wideband current-feedback op amp with a highly
linear output stage. Requiring only 12.9mA/channel
supply current, the OPA3695 achieves a 900MHz
small-signal bandwidth (G = +2V/V); the high slew
rate capability of up to 4300V/µs supports a 600MHz
2V_PP large signal into a 100Ω load. The low output
headroom of 1V from either supply in a very
high-speed amplifier gives very good single +5V
operation. The OPA3695 delivers a 2V_PP swing with
greater than 400MHz bandwidth operating on a single
+5V supply. The primary advantage of a
current-feedback video buffer (as opposed to a
slew-enhanced, low-gain, stable voltage-feedback
implementation) is a higher slew rate with lower
quiescent power and output noise.
Figure 35 shows the dc-coupled, noninverting, dual
power-supply circuit configuration used as the basis
for the ±5V Electrical Characteristics table and
Typical Characteristics curves. For test purposes, the
input impedance is set to 50Ω with a resistor to
ground; the output impedance is set to 50Ω with a
series output resistor. Voltage swings reported in the
specifications 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 35, the
total effective amplifier loading is 100Ω || (R_F + R_G) .
For example, with a gain of +2V/V with R_F and R_G
equal to 604Ω, the equivalent amplifier loading is
100Ω || 1208Ω = 92.3Ω. The disable control line
(DIS) is typically left open to ensure normal amplifier
operation. Note that while most of the information
presented in this data sheet was characterized with
100Ω loading, performance with a standard video
loading of 150Ω has negligible impact on
performance. Any changes in performance are
typically improved over 100Ω loading because of
lower output current demands.
+5V
50W Source
V_I
50W
0.1mF
6.8mF
1/3
OPA3695
R_F
DIS
50W
V_O
50W Load
R_G
0.1mF
6.8mF
-5V
Figure 35. DC-Coupled, Noninverting,
Bipolar-Supply, Specification and Test Circuit
Figure 36 illustrates the dc-coupled, inverting
configuration used as the basis of the Inverting
Typical Characteristic curves. Inverting operation
offers several performance benefits. Since there is no
common-mode signal across the input stage, the slew
rate for inverting operation is higher and the distortion
performance is slightly improved. An additional input
resistor, R_M, is included in Figure 36 to set the input
impedance equal to 50Ω. The parallel combination of
R_M and R_G sets the input impedance. Both the
noninverting and inverting applications of Figure 35
and Figure 36 benefit from optimizing the feedback
resistor (R_F) value for bandwidth (see the discussion
in the Gain Setting section). The typical design
sequence is to select the R_F value for best
bandwidth, set R_G for the gain, and then set R_M for
the desired input impedance. As the gain increases
for the inverting configuration, a point is reached
where R_G equals 50Ω and R_M is removed; thus, the
input match is set by R_G only. With R_G fixed to
achieve an input match to 50Ω, R_F is simply
increased to increase gain. This approach, however,
quickly reduces the achievable bandwidth at such
high gains. For gains greater than 10V/V,
noninverting operation is recommended to maintain
broader bandwidth.
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): OPA3695
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