OPA3693_07 Datasheet, PDF (16/31 Pages) Burr-Brown (TI) – Triple, Ultra-Wideband, Fixed-Gain, VIDEO BUFFER with Disable

OPA3693
SBOS353 – DECEMBER 2006
Figure 44 shows the DC-coupled, gain of –1V/V
buffer configuration 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 OPA3693 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 44 provides the dc
operating voltage. This application of the OPA3693
provides a very broadband, high-output, signal
inverter.
+5V
0.1mF
6.8mF
50W Source
V_I
R_G
300W
R_M
60.4W
1/3
OPA3693
R_F
300W
DIS
V_O ^50W
50W Load
0.1mF
6.8mF
-5V
Figure 44. DC-Coupled, G = –1V/V,
Bipolar-Supply Specification and Test Circuit
SINGLE-SUPPLY OPERATION
The OPA3693 may be used over a single-supply
range of +3.5V to +12V. Though not a rail-to-rail
output design, the OPA3693 requires minimal input
and output voltage headroom compared to other
very-wideband video buffer amplifiers. As illustrated
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in the single +5V Typical Characteristic curves, the
OPA3693 provides > 300MHz bandwidth driving a
3V_PP swing into a 100Ω load. 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 45 shows the AC-coupled, gain
of +2V/V, video buffer circuit used as the basis for
the Electrical Characteristics table and Typical
Characteristics curves. The circuit of Figure 45
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 more than 85mA output current.
A demanding 100Ω load to a midpoint bias is used in
this characterization circuit. The new output stage
used in the OPA3693 can deliver large bipolar output
current into this midpoint load with minimal crossover
distortion, as illustrated by the +5V supply,
3rd-harmonic distortion plots.
50W Source
1000pF
V_I
60.4W
604W
604W
+V_S +5V
0.1mF
6.8mF
DIS
1/3
V_O 100W
OPA3693
V_S/2
R_F
300W
R_G
300W
1000pF
Figure 45. AC-Coupled, G = +2V/V, Single-Supply
Specification and Test Circuit
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