OPA2695 Datasheet, PDF (19/40 Pages) Texas Instruments – Dual, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2695
www.ti.com..................................................................................................................................................................................................... SBOS354 – APRIL 2008
APPLICATIONS INFORMATION
WIDEBAND CURRENT-FEEDBACK
OPERATION
The OPA2695 gives a new level of performance in
wideband current-feedback op amps. Nearly constant
ac performance over a wide gain range, along with
2900V/µs slew rate, gives a lower power and cost
solution for high-intercept IF amplifier requirements.
While optimized at a gain of +8V/V (12dB to a
matched 50Ω load) to give 450MHz bandwidth,
applications from gains of 1 to 40 can be supported.
At gains above 20, the signal bandwidth starts to
decrease, but continues to exceed 180MHz up to a
gain of 40V/V (26dB to a matched 50Ω load). Single
+5V supply operation is also supported with similar
bandwidths but reduced output power capability. For
lower speed (< 250MHz) requirements with higher
output power, consider the OPA2691.
Figure 68 shows the dc-coupled, gain of +8V/V, dual
power-supply circuit used as the basis of the ±5V
Electrical Characteristics and Typical Characteristics.
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 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 68, the total effective load is 100Ω ||
458Ω = 82Ω. The disable control line (DIS) is typically
left open to get normal amplifier operation. The
disable line must be asserted low to shut off the
OPA2695. Figure 68 includes one optional
component. In addition to the usual power-supply
decoupling capacitors to ground, a 0.01µF capacitor
is included between the two power-supply pins. In
practical printed circuit board (PCB) layouts, this
optional added capacitor typically improves the
2nd-harmonic distortion performance by 3dB to 6dB
for bipolar supply operation.
+5V
50W Source
V_I
50W
0.1mF
6.8mF
1/2
OPA2695
DIS
50W Load
V_O ^50W
Optional
0.01mF
R_F
402W
R_G
56.2W
0.1mF
6.8mF
-5V
Figure 68. DC-Coupled, G = +8V/V, Bipolar Supply
Specifications and Test Circuit
Figure 69 shows the dc-coupled, gain of –8V/V, dual
power-supply circuit used as the basis of the Inverting
Typical Characteristic curves. Inverting operation
offers several performance benefits. Because 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_T, is included in Figure 69
to set the input impedance equal to 50Ω. The parallel
combination of R_T and R_G set the input impedance.
Both the noninverting and inverting applications of
Figure 68 and Figure 69 benefit from optimizing the
feedback resistor (R_F) value for bandwidth (see the
discussion in Setting Resistor Values to Optimize
Bandwidth). The typical design sequence is to select
the R_F value for best bandwidth, set R_G for the gain,
then set R_T for the desired input impedance. As the
gain increases for the inverting configuration, a point
is reached where R_G equals 50Ω, where R_T is
removed and 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 increase,
however, quickly reduces the achievable bandwidth,
as shown by the inverting gain of –16V/V frequency
response in the Typical Characteristic curves. For
gains greater than 10V/V (14dB at the matched load),
noninverting operation is recommended to maintain
broader bandwidth.
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): OPA2695
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