OPA3681 Datasheet, PDF (12/21 Pages) Burr-Brown (TI) – Triple Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

TRIPLE ADC BUFFER CHANNEL
The OPAx681 family is ideally suited to single supply,
wideband ADC driving. A current feedback op amp is ideal
where high gains with high bandwidths are required. The
wide 3Vp-p output swing with over 150MHz full power
bandwidth on a single +5V supply is well suited to the
2Vp-p input range commonly required from modern CMOS
pipelined ADCs. Three channels of very high speed digitizer
channels are shown in Figure 3 using the OPA3681 driving
three ADS831s (8-bit, 80Msps CMOS converters). Each
input is AC-coupled into a 50Ω gain resistor that also will
act as a 50Ω impedance match at high frequencies. The
amplifier’s inputs and outputs are centered on the ADC
common-mode input voltage by tying each converter’s V_CM
to the non-inverting inputs of the amplifier. This V_CM acts as
the swing midpoint for the input to the converter. Since the
ADS831 can operate with differential inputs, driving into
the IN input will give a net non-inverting signal channel
even with the amplifiers operating at an inverting gain of
–6. The other input to the ADS831 is tied to this V_CM as well
to give an input signal midpoint equal to V_CM. The 300Ω
feedback resistor will be the output load in this configura-
tion. Harmonic distortion for the OPA3681 will not degrade
the converter’s SFDR performance in this application.
Power Supply
De-Coupling
Not Shown
0.1µF
+5V
1/3
OPA3681
0.1µF 50Ω
V₁
300Ω
300Ω
22Ω
47pF
IN
V_CM
ADS831
IN 8-Bit
80Msps
300Ω
IN
V_CM
WIDEBAND RGB MULTIPLEXER
The OPA3681 is ideally suited to implementing a simple,
very wideband, 2x1 RGB multiplexer. This simple “wired-
OR video multiplexer” can be easily implemented using the
circuit shown in Figure 4.
This circuit uses two OPA3681s where each package ac-
cepts the three RGB component video signals from one of
two possible sources. Each non-inverting input is terminated
+5V
V_DIS
+5V
R1
75Ω
U1
1/3
OPA3681
Power Supply
De-Coupling Not Shown
82.5Ω
V_OUT Red
340Ω
402Ω
75Ω Line
G1
75Ω
1/3
OPA3681
340Ω
402Ω
82.5Ω
V_OUT Green
75Ω Line
B1
75Ω
1/3
OPA3681
340Ω
402Ω
R2
75Ω
–5V
+5V
U2
1/3
OPA3681
82.5Ω
82.5Ω
V_OUT Blue
75Ω Line
0.1µF
1/3
OPA3681
0.1µF 50Ω
V₁
300Ω
0.1µF
1/3
OPA3681
0.1µF 50Ω
V₁
300Ω
22Ω
47pF
ADS831
IN 8-Bit
80Msps
300Ω
22Ω
47pF
IN
V_CM
ADS831
IN 8-Bit
80Msps
340Ω
402Ω
G2
75Ω
1/3
OPA3681
340Ω
402Ω
B2
1/3
75Ω
OPA3681
340Ω
402Ω
–5V
82.5Ω
82.5Ω
FIGURE 3. ADC Driver.
®
OPA3681
FIGURE 4. Wideband 2x1 RGB Multiplexer.
12