+5V
2kΩ
VDIS
+5V
Video 1
DIS
OPA681
75Ω
82.5Ω
–5V
340Ω
402Ω
402Ω
75Ω Cable
RG-59
340Ω
+5V
82.5Ω
OPA681
Video 2
DIS
75Ω
–5V
2kΩ
FIGURE 4. Two-Channel Video Multiplexer.
The section on Disable Operation shows the turn-on and
turn-off switching glitches using a grounded input for a
single channel is typically less than ±50mV. Where two
outputs are switched (as shown in Figure 6), the output line
is always under the control of one amplifier or the other due
to the “make-before-break” disable timing. In this case, the
switching glitches for two 0V inputs drop to <20mV.
+5V
Power Supply
de-coupling not shown
5kΩ
5kΩ
DIS
50Ω
1µF
VO
OPA681
50Ω Load
SINGLE-SUPPLY “IF” AMPLIFIER
50Ω Source
The high bandwidth provided by the OPA681 while operat-
ing on a single +5V supply lends itself well to IF amplifier
applications. One of the advantages of using an op amp like
the OPA681 as an IF amplifier is that precise signal gain is
achieved along with much lower 3rd-order intermodulation
versus quiescent power dissipation. In addition, the OPA681
in the SOT23-6 package offers a very small package with a
power shutdown feature for portable applications. One con-
cern with using op amps for an IF amplifier is their relatively
high noise figures. It is sometimes suggested that an opti-
mum source resistance can be used to minimize op amp
noise figure. Adding a resistor to reach this optimum value
may improve noise figure, but will actually decrease the
signal-to-noise ratio. A more effective way to move towards
an optimum source impedance is to bring the signal in
through an input transformer. Figure 5 shows an example
that is particularly useful for the OPA681.
RG
200Ω
RF
600Ω
1:2
VI
VO
VI
= 3V/V (9.54dB)
0.1µF
FIGURE 5. Low Noise, Single Supply, IF Amplifier.
Bringing the signal in through a step up transformer to the
inverting input gain resistor has several advantages for the
OPA681. First, the decoupling capacitor on the non-invert-
ing input eliminates the contribution of the non-inverting
input current noise to the output noise. Secondly, the non-
inverting input noise voltage of the op amp is actually
®
13
OPA681
BB [ BURR-BROWN CORPORATION ]
