Datasheet

OPA693
14
SBOS285A
www.ti.com
APPLICATION INFORMATION
WIDEBAND BUFFER OPERATION
The OPA693 gives the exceptional AC performance of a
wideband current-feedback op amp with a highly linear
output stage. It features internal R
F
and R
G
resistors, making
it a simple matter to select a gain of +2, +1 or –1 with no
external resistors. Requiring only 13mA supply current, the
OPA693’s output swings to within 1V of either supply with
> 700MHz small signal bandwidth and > 300MHz delivering
7V
PP
into a 100Ω load. This low output headroom in a very
high-speed amplifier gives remarkable single +5V operation.
The OPA693 delivers 2V
PP
swing with > 500MHz bandwidth
operating on a single +5V supply. The primary advantage of
a current-feedback fixed gain video buffer, as opposed to a
slew-enhanced low-gain stable voltage-feedback implemen-
tation, is a higher slew rate with lower quiescent power and
output noise.
Figure 1 shows the DC-coupled, gain of +2V/V, 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 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 || 600Ω = 85.7Ω. The disable control line (
DIS ) is
typically left open to ensure normal amplifier operation. In
addition to the usual power supply decoupling capacitors to
ground, a 0.01µF capacitor can be included between the two
power-supply pins. This optional added capacitor will typi-
cally improve the 2nd harmonic distortion performance by
3dB to 6dB.
Figure 2 shows the DC-coupled, gain of +1V/V buffer con-
figuration used as a starting point for the gain of +1V/V
Typical Characteristic curves. In this case, the inverting input
resistor, R
G
, is left open giving a very broadband gain of +1
performance. While the test circuit shows a 50Ω input resis-
tor, a buffer application is typically transforming from a
source that cannot drive a heavy load to a 100Ω load, such
as shown in Figure 2. The noninverting input impedance of
the OPA693 is typically 100kΩ || 2pF. Driving directly into
the noninverting input will provide this very light load to the
source. However, the source must still provide the noninverting
input bias current required by the input stage to operate. An
alternative approach to a gain of +1 buffer is described in the
Wideband Unity Gain Buffers section of this data sheet.
OPA693
+5V
−5V
50Ω Load
50Ω
50Ω
50Ω Source
R
G
300Ω
R
F
300Ω
+
6.8µF0.1µF
+
6.8µF0.1µF
V
I
V
O
DIS
Figure 1. DC-Coupled, G = +2, Bipolar-Supply, Specification
and Test Circuit.
Figure 3 shows the DC-coupled, gain of –1V/V buffer con-
figuration used as a starting point for the gain of –1V/V
Typical Characteristic curves. The input impedance is set to
50Ω using the parallel combination of an external 60.4Ω
resistor and the internal 300Ω R
G
resistor. The noninverting
input is tied directly to ground. Since the internal design for
the OPA693 is current-feedback, trying to get improved DC
accuracy by including a resistor on the noninverting input to
ground is ineffective. Using a direct short to ground on the
noninverting input reduces both the contribution of the DC
bias current and noise current to the output error. While the
external 60.4Ω is used here to match to the 50Ω source from
the test equipment, the maximum input impedance in this
configuration is limited to the 300Ω R
G
resistor even with the
R
M
resistor removed. Unlike the noninverting unity gain
buffer application, removing R
M
does not strongly impact the
DC operating point because the short on the noninverting
input of Figure 3 provides the DC operating voltage. This
application of the OPA693 provides a very broadband, high-
output, signal inverter.
Figure 2. DC-Coupled, G = +1V/V, Bipolar-Supply, Specifica-
tion and Test Circuit.
OPA693
+5V
−5V
Open
50Ω Load
50Ω
50Ω
50Ω Source
R
G
300Ω
R
F
300Ω
+
6.8µF0.1µF
+
6.8µF0.1µF
V
I
DIS
V
O