Datasheet

OPA695
13
SBOS293G
www.ti.com
FIGURE 1. DC-Coupled, G = +8V/V, Bipolar Supply Speci-
fications and Test Circuit.
FIGURE 2. DC-Coupled, G = –8V/V, Bipolar Supply Speci-
fications and Test Circuit.
APPLICATIONS INFORMATION
WIDEBAND CURRENT FEEDBACK OPERATION
The OPA695 gives a new level of performance in wideband
current feedback op amps. Nearly constant AC performance
over a wide gain range, along with 4300V/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. As a gain of +2
video line driver, the bandwidth extends to 1.4GHz with
a slew rate to support the highest pixel rates. At gains
above 20, the signal bandwidth starts to decrease, but still
exceeds 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
powers, consider the OPA691.
Figure 1 shows the DC-coupled, gain of +8V/V, dual power
supply circuit used as the basis of the ±5V Specifications and
Typical Characteristic 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 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
1, the total effective load will be 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 OPA695. One optional component is included in
Figure 1. 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 PC board layouts, this
optional added capacitor will typically improve the 2nd-
harmonic distortion performance by 3dB to 6dB for bipolar
supply operation.
Figure 2 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 per-
formance 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
T
, is included in Figure 2 to set
the input impedance equal to 50Ω. The parallel combination
of R
T
and R
G
set the input impedance. Both the non-inverting
and inverting applications of Figures 1 and 2 will 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 will be reached where R
G
will
equal 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 will, however,
quickly reduce the achievable bandwidth, as shown by the
inverting gain of –16 frequency response in the Typical
Characteristic curves. For gains > 10V/V (14dB at the matched
load), noninverting operation is recommended to maintain
broader bandwidth.
OPA695
+5V
+V
S
–V
S
DIS
–5V
50Ω Load
50Ω
20Ω
R
T
562Ω
R
G
54.9Ω
+
6.8µF0.1µF
+
6.8µF0.1µF
Optional
0.01µF
V
I
50Ω Source
R
F
442Ω
V
O
OPA695
+5V
DIS
–5V
50Ω Load
50Ω
50Ω
V
I
50Ω Source
R
G
56.2Ω
R
F
402Ω
V
O
+
6.8µF0.1µF
+
6.8µF0.1µF
Optional
0.01µF