Datasheet

OPA3691
13
www.ti.com
SBOS227E
TRIPLE ADC BUFFER CHANNEL
The OPAx691 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 band-
width 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 OPA3691 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 noninverting
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
noninverting signal channel even with the amplifiers operat-
ing 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 configuration. Harmonic distortion for the OPA3691
will not degrade the converter’s SFDR performance in this
application.
WIDEBAND RGB MULTIPLEXER
The OPA3691 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 OPA3691s where each package accepts
the three RGB component video signals from one of two
possible sources. Each noninverting input is terminated in 75Ω
FIGURE 4. Wideband 2x1 RGB Multiplexer.
FIGURE 3. Triple-Channel ADC Driver.
1/3
OPA3691
22Ω
300Ω
50Ω
0.1µF
300Ω
0.1µF
47pF
V
1
ADS831
8-Bit
80MSPS
IN
V
CM
IN
1/3
OPA3691
22Ω
300Ω
50Ω
0.1µF
300Ω
0.1µF
47pF
V
1
ADS831
8-Bit
80MSPS
IN
V
CM
IN
1/3
OPA3691
22Ω
300Ω
50Ω
0.1µF
300Ω
0.1µF
47pF
V
1
ADS831
8-Bit
80MSPS
IN
V
CM
IN
+5V
Power-supply
decoupling not shown.
1/3
OPA3691
340Ω 402Ω
75Ω
82.5Ω
V
OUT
Red
75Ω Line
R1
+5V
+5V
–5V
1/3
OPA3691
340Ω 402Ω
75Ω
82.5Ω
V
OUT
Green
75Ω Line
G1
1/3
OPA3691
340Ω 402Ω
75Ω
82.5Ω
V
OUT
Blue
75Ω Line
B1
1/3
OPA3691
340Ω 402Ω
75Ω
82.5Ω
R2
+5V
–5V
1/3
OPA3691
340Ω 402Ω
75Ω
82.5Ω
G2
1/3
OPA3691
340Ω 402Ω
75Ω
82.5Ω
B2
V
DIS
U1
U2
Power-supply
decoupling not shown.