Datasheet

APPLICATIONS INFORMATION
WIDEBAND CURRENT-FEEDBACK
1/2
OPA2695
+5V
DIS
-5V
50 LoadW
50W
50W
V
I
50 SourceW
R
G
56.2W
R
F
402W
V
O
+
6.8 Fm0.1 Fm
+
6.8 Fm0.1 Fm
Optional
0.01 Fm
OPA2695
www.ti.com ...................................................................................................................................................... SBOS354A – APRIL 2008 – REVISED AUGUST 2008
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/ µ s 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 50 Ω 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 50 Ω 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 50 Ω
Figure 68. DC-Coupled, G = +8V/V, Bipolar Supply
with a resistor to ground and the output impedance is
Specifications and Test Circuit
set to 50 Ω with a series output resistor. Voltage
swings reported in the specifications are taken
Figure 69 shows the dc-coupled, gain of – 8V/V, dual
directly at the input and output pins while load powers
power-supply circuit used as the basis of the Inverting
(dBm) are defined at a matched 50 Ω load. For the
Typical Characteristic curves. Inverting operation
circuit of Figure 68 , the total effective load is 100 Ω ||
offers several performance benefits. Because there is
458 Ω = 82 Ω . The disable control line ( DIS) is typically
no common-mode signal across the input stage, the
left open to get normal amplifier operation. The
slew rate for inverting operation is higher and the
disable line must be asserted low to shut off the
distortion performance is slightly improved. An
OPA2695. Figure 68 includes one optional
additional input resistor, R
T
, is included in Figure 69
component. In addition to the usual power-supply
to set the input impedance equal to 50 Ω . The parallel
decoupling capacitors to ground, a 0.01 µ F capacitor
combination of R
T
and R
G
set the input impedance.
is included between the two power-supply pins. In
Both the noninverting and inverting applications of
practical printed circuit board (PCB) layouts, this
Figure 68 and Figure 69 benefit from optimizing the
optional added capacitor typically improves the
feedback resistor (R
F
) value for bandwidth (see the
2nd-harmonic distortion performance by 3dB to 6dB
discussion in Setting Resistor Values to Optimize
for bipolar supply operation.
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 50 Ω , 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 50 Ω , 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.
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