Datasheet

OPA693
15
SBOS285A
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SINGLE-SUPPLY OPERATION
The OPA693 may be used over a single-supply range of +5V
to +12V. Though not a rail-to-rail output design, the OPA693
requires minimal input and output voltage headroom com-
pared to other very-wideband video buffer amplifiers. As
shown in the single +5V Typical Characteristic curves, the
OPA693 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 4 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 4 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
1.7V of either supply pin, giving a 1.6V
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 +1, 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 90mA
output current. A demanding 100Ω load to a midpoint bias is
used in this characterization circuit. The new output stage
used in the OPA693 can deliver large bipolar output current
into this midpoint load with minimal crossover distortion, as
shown by the +5V supply, 3rd-harmonic distortion plots.
While the circuit of Figure 4 shows +5V single-supply opera-
tion, this same circuit may be used for single supplies
ranging as high as +12V nominal. The noninverting input bias
resistors are relatively low in Figure 4 to minimize output DC
offset due to noninverting input bias current. At higher signal-
supply voltage, these should be increased to limit the added
supply current drawn through this path.
Figure 5 shows the AC-coupled, G = +1V/V, single-supply
specification and test circuit. In this case, the gain setting
resistor, R
G
, is simply left open to get a gain of +1V for AC
signals. Once again, the noninverting input is DC biased at
mid-supply, putting that same V
S
/2 at the output pin. The
signal is AC-coupled into this midpoint with an added termi-
nation resistor on the source side of the blocking capacitor.
Figure 3. DC-Coupled, G = –1V/V, Bipolar-Supply Specifica-
tion and Test Circuit.
OPA693
+5V
DIS
50Ω
V
O
R
F
300Ω
R
G
300Ω
R
M
60.4Ω
+
6.8µF0.1µF
50Ω Load
−5V
+
6.8µF0.1µF
50Ω Source
V
I
Figure 4. AC-Coupled, G = +2V/V, Single-Supply Specifica-
tion and Test Circuit.
OPA693
+5V
+V
S
DIS
V
S
/2
604Ω
100ΩV
O
V
I
604Ω
R
G
300Ω
R
F
300Ω
1000pF
1000pF
+
6.8µF0.1µF
50Ω Source
60.4Ω
Figure 5. AC-Coupled, G = +1V/V, Single-Supply Specifica-
tion and Test Circuit.
OPA693
Open
V
S
/2
V
O
60.4Ω
100Ω
50Ω Source
604Ω
604Ω
R
G
300Ω
R
F
300Ω
+
6.8µF0.1µF
V
I
DIS
1000pF
+5V
V
S