OPA3684 Datasheet, PDF (14/32 Pages) Burr-Brown (TI) – Low-Power, Triple Current-Feedback OPERATIONAL AMPLIFIER With Disable

LOW-POWER RGB MUX/LINE DRIVER
Using the shutdown feature, two OPA3684s can provide an
easy low-power way to select one of two possible RGB
sources for moderate resolution monitors. Figure 6 shows a
recommended circuit where each of the color outputs are
combined in a way that provides a net gain of 1 to the
matched 75Ω load with a 75Ω output impedance. This brings
the two outputs for each color together through a 78.7Ω
resistor with a slightly > 2 gain provided by the amplifiers.
+5V
V_DIS
+5V
Power Supply
De-Coupling Not Shown
R1
75Ω
U1
1/3
OPA3684
78.7Ω
V_OUT Red
681Ω
806Ω
75Ω Line
G1
75Ω
1/3
OPA3684
681Ω
806Ω
78.7Ω
V_OUT Green
75Ω Line
B1
75Ω
1/3
OPA3684
681Ω
806Ω
R2
75Ω
–5V
+5V
U2
1/3
OPA3684
681Ω
806Ω
78.7Ω
78.7Ω
V_OUT Blue
75Ω Line
G2
75Ω
1/3
OPA3684
681Ω
806Ω
78.7Ω
B2
1/3
75Ω
OPA3684
78.7Ω
681Ω
806Ω
–5V
FIGURE 6. Wideband 2x1 RGB Multiplexer.
When one channel is shutdown, the feedback network is still
present, slightly attenuating the signal and combining in
parallel with the 78.7Ω to give a 75Ω source impedance.
Since the OPA3684 does not disable quickly, this approach
is not suitable for pixel-by-pixel multiplexing—however, it
does provide an easy way to switch between two possible
RGB sources. The output swing provided by the active
channel will divide back through the inactive channel feed-
back to appear at the inverting input of the OFF channel. To
retain good pulse fidelity, or low distortion, this divided down
output signal at the inverting inputs of the OFF channels, plus
the OFF channel input signals, should not exceed 0.7Vp-p.
As the signal across the buffers of the inactive channels
exceeds 0.7Vp-p, diodes across the inputs begin to turn on
causing a nonlinear load to the active channel. This will
degrade signal purity under those conditions.
LOW-POWER, FLEXIBLE GAIN, DIFFERENTIAL
RECEIVER
The 3 channels available in the OPA3684 can be applied to
a very flexible differential to single-ended receiver. Since the
bandwidth does not depend on the gain setting, the gain
setting element of Figure 7 (R_G) can be adjusted over a wide
range with minimal impact on resulting bandwidth. Fre-
quency-response shaping elements may be included in R_G
as well to provide line equalization or filtering in the final
output signal.
+5
V₁
1/3
OPA3684
–5
806Ω
R_G
806Ω
402Ω
402Ω
+5
1/3
(1 + 2(806Ω)/R_G) (V₁ – V₂)
OPA3684
–5
806Ω
+5
806Ω
1/3
OPA3684
V₂
–5 ^{High-Speed INA (>120MHz)}
FIGURE 7. Low-Power, Wide Gain Range, Differential Receiver.
The first two amplifiers provide the differential gain function
with a common-mode gain of 1. The second amplifier per-
forms the differencing function to remove the common-mode
(referencing the output to ground if the 402Ω resistor is
grounded) and providing a differential gain of 1. The resistors
have been scaled to provide the same output loading on
each first stage amplifier. Typical bandwidths for the circuit of
Figure 7 exceed 120MHz.
14
OPA3684
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