Datasheet

OPA684
13
SBOS219D
www.ti.com
voltage bias. The input voltage can swing to within 1.25V of
either supply pin, giving a 2.5V
PP
input signal range centered
between the supply pins. The input impedance of Figure 3 is
set to give a 50Ω input match. If the source does not require
a 50Ω match, remove R
M
and drive directly into the blocking
capacitor. The source will then see the 5kΩ load of the
biasing network. The gain resistor (R
G
) is AC coupled, giving
the circuit a DC gain of +1, which puts the non-inverting input
DC bias voltage (2.5V) on the output as well. The feedback
resistor value has been adjusted from the bipolar supply
condition to re-optimize for a flat frequency response in +5V
only, gain of +2, operation. On a single +5V supply, the
output voltage can swing to within 1.0V of either supply pin
while delivering more than 70mA output current giving 3V
output swing into 100Ω (8dBm maximum at a matched 50Ω
load). The circuit of Figure 3 shows a blocking capacitor
driving into a 50Ω output resistor then into a 50Ω load.
Alternatively, the blocking capacitor could be removed if the
load is tied to a supply midpoint or to ground if the DC current
required by the load is acceptable.
The circuits of Figure 3 and 4 show single-supply operation
at +5V. These same circuits may be used up to single
supplies of +12V with minimal change in the performance of
the OPA684.
LOW-POWER VIDEO LINE DRIVER APPLICATIONS
For low-power, video line driving, the OPA684 provides the
output current and linearity to support multiple load compos-
ite video signals. Figure 5 shows a typical ±5V supply video
line driver application. The improved 2nd-harmonic distortion
of the CFB
plus
architecture, along with the OPA684’s high
output current and voltage, gives exceptional differential gain
and phase performance for a low-power solution. As the
Typical Characteristics show, a single video load shows a
dG/dP of 0.04%/0.02°. Multiple loads may also be driven with
< 0.1%/0.1° dG/dP for up to 4 parallel video loads, where the
amplifier is driving an equivalent load of 37.5Ω.
R
F
1.3kΩ
OPA684
+5V
DIS
50Ω
50Ω Load
50Ω Source
0.1µF 6.8µF
+
10kΩ
10kΩ
R
M
50Ω
R
G
1.3kΩ
0.1µF
0.1µF
0.1µF
V
I
FIGURE 3. Non-inverting Single-Supply Test and Character-
ization Circuit.
Figure 4 shows the AC coupled, single +5V supply, gain of
–1V/V circuit configuration used as a basis for the +5V
Typical Characteristics. In this case, the midpoint DC bias on
the non-inverting input is also decoupled with an additional
0.1µF decoupling capacitor. This reduces the source imped-
ance at higher frequencies for the non-inverting input bias
current noise. This 2.5V bias on the non-inverting input pin
appears on the inverting input pin and, since R
G
is DC
blocked by the input capacitor, will also appear at the output
pin. One advantage to inverting operation is that since there
is no signal swing across the input stage, higher slew rates
and operation to even lower supply voltages is possible. To
retain a 1V
PP
output capability, operation down to a 3V
supply is allowed. At a +3V supply, the input stage is
saturated, but for the inverting configuration of a current
feedback amplifier, wideband operation is retained even
under this condition.
FIGURE 4. Inverting Single-Supply Test and Characteriza-
tion Circuit.
R
F
1.3kΩ
OPA684
+5V
DIS
50Ω
50Ω Load
50Ω Source
0.1µF
0.1µF
6.8µF
+
R
G
1.3kΩ
10kΩ
10kΩ
0.1µF
V
I
0.1µF
R
M
52.3Ω
1kΩ
OPA684
+5V
DIS
–5V
75Ω
75Ω
1kΩ
75Ω Load
Supply Decoupling not shown.
Coax
VIDEO
IN
FIGURE 5. Gain of +2 Video Cable Driver.