OPA3695_14 Datasheet, PDF (14/39 Pages) Texas Instruments – Triple, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3695
SBOS355A – APRIL 2008 – REVISED SEPTEMBER 2008 ............................................................................................................................................... www.ti.com
+5V
0.1mF
6.8mF
50W Source
R_G
V_I
R_M
1/3
OPA3695
R_F
DIS
V_O ^50W
50W Load
0.1mF
6.8mF
-5V
Figure 36. DC-Coupled, Inverting, Bipolar-Supply,
Specification and Test Circuit
Notice that in this configuration (shown in Figure 36),
the noninverting input is tied directly to ground.
Because the internal design for the OPA3695 is
current-feedback, trying to achieve 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 the noise
current to the output error. While the external R_M is
used here to match with the 50Ω source from the test
equipment, the input impedance in this configuration
is limited to the R_G resistor. Removing R_M does not
strongly impact the dc operating point because the
short on the noninverting input of Figure 36 provides
the dc operating voltage. This application of the
OPA3695 provides a very broadband, high-output
signal inverter.
SINGLE-SUPPLY OPERATION
The OPA3695 may be used over a single-supply
range of +3.5V to +12V. Though not a rail-to-rail
output design, the OPA3695 requires minimal input
and output voltage headroom compared to other
very-wideband video buffer amplifiers. The key
requirement of broadband single-supply operation is
to maintain input and output signal swings within the
useable voltage ranges at both the input and the
output.
The circuit of Figure 37 shows the single-supply
ac-coupled, gain of +8V/V, video buffer circuit used
as the basis for the Electrical Characteristics table
and Typical Characteristics curves. The circuit of
Figure 37 establishes an input midpoint bias using a
simple resistive divider from the +5V supply (two
604Ω resistors). The input signal is then ac-coupled
into this midpoint voltage bias. The input voltage can
swing to within 1.6V of either supply pin, giving a
1.8V_PP input signal range centered between the
supply pins. The input impedance matching resistor
(60.4Ω) used for testing is adjusted to give a 50Ω
input match when the parallel combination of the
biasing divider network is included. The gain resistor
(R_G) is ac-coupled, giving the circuit a dc gain of
+1V/V, which puts the input dc bias voltage (2.5V) on
the output as well. Again, on a single +5V supply, the
output voltage can swing to within 1V of either supply
pin while delivering ±90mA output current. A
demanding 100Ω load to a midpoint bias is used in
this characterization circuit. The new output stage
used in the OPA3695 can deliver large bipolar output
current into this midpoint load with minimal crossover
distortion, as illustrated by the +5V supply,
third-harmonic distortion plots.
+V_S +5V
50W Source
0.1mF
V_I
60.4W
604W
604W
0.1mF
6.8mF
DIS
1/3
V_O 100W
OPA3695
V_S/2
R_F
348W
R_G
49.9W
0.1mF
Figure 37. AC-Coupled, G = +8V/V, Single-Supply
Specification and Test Circuit
While the circuit of Figure 37 shows +5V
single-supply operation, this same circuit may be
used for single supplies that range as high as +12V
nominal. The noninverting input bias resistors are
relatively low in Figure 37 to minimize output dc offset
as a result of noninverting input bias current. At
higher signal-supply voltages, these resistors should
be increased in order to limit the added supply
current drawn through this path.
14
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