OPA3684 Datasheet, PDF (13/26 Pages) Burr-Brown (TI) – Low-Power, Triple Current-Feedback OPERATIONAL AMPLIFIER With Disable

directly into the blocking capacitor. The source will then see
the 5kΩ load of the biasing network as a load. The gain
resistor (R_G) is AC-coupled, giving the circuit a DC gain of +1,
which puts the noninverting input DC bias voltage (2.5V) on
the output as well. The feedback resistor value has been
adjusted from the bipolar ±5V 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—easily giving a 3Vp-p output
swing into 100Ω (8dBm maximum at the 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 then required
by the load is acceptable.
+5V
50Ω Source _0.1µF
V_I
R_M
50Ω
10kΩ
+
0.1µF 6.8µF
DIS
1/3
0.1µF _50Ω
10kΩ _OPA3684
50Ω Load
R_F
1kΩ
R_G
1kΩ
0.1µF
FIGURE 3. AC-Coupled, G = +2V/V, Single-Supply Specifi-
cations and Test Circuit.
Figure 4 shows the AC-coupled, single +5V supply, gain of
–1V/V circuit configuration used as a basis for the inverting
+5V only Typical Characteristics for each channel. In this
case, the midpoint DC bias on the noninverting input is also
decoupled with an additional 0.1µF capacitor. This reduces
the source impedance at higher frequencies for the
noninverting input bias current noise. This 2.5V bias on the
noninverting 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 opera-
tion 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 1Vp-p 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 re-
tained even under this condition.
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 OPA3684.
+5V
0.1µF
10kΩ
0.1µF
DIS
1/3
10kΩ _OPA3684
+
6.8µF
0.1µF _50Ω
50Ω Load
50Ω Source
V_I
0.1µF
R_G
1.0kΩ
R_M
52.3Ω
R_F
1.0kΩ
FIGURE 4. AC-Coupled, G = –1V/V, Single-Supply Specifi-
cations and Test Circuit.
LOW-POWER, VIDEO LINE DRIVER APPLICATIONS
For low-power, video line driving, the OPA3684 provides the
output current and linearity to support 3 channels of either
single video lines, or up to 4 video lines in parallel on each
output. Figure 5 shows a typical ±5V supply video line driver
application where only one channel is shown and only a
single line is being driven. The improved 2nd-harmonic
distortion of the CFB_PLUS architecture, along with the
OPA3684’s high output current and voltage, gives excep-
tional 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 be driven on each output, with minimal x-talk, while the
dG/dP is still < 0.1%/0.1° for up to 4 parallel video loads. The
slew rate and gain of 2 bandwidth are also suitable to
moderate resolution RGB applications.
VIDEO_IN
75Ω
+5V
DIS
OPA3684
Supply decoupling not shown.
75Ω _{Coax 75Ω Load}
1kΩ
1kΩ
–5V
FIGURE 5. Noninverting Differential I/O Amplifier.
OPA3684
13
SBOS241A
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