OPA681N BURR-BROWN [Burr-Brown Corporation], OPA681N Datasheet - Page 13

• Manufacturer Part Number: OPA681N
• Description: Wideband, Current Feedback OPERATIONAL AMPLIFIER With Disable
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

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.
SINGLE-SUPPLY “IF” AMPLIFIER
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.
V
DIS
Video 1
Video 2
+5V
75
75
340
340
2k
2k
13
OPA681
OPA681
+5V
–5V
+5V
–5V
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
50 Source
DIS
DIS
V
I
402
402
1:2
0.1µF
1µF
200
R
G
5k
5k
82.5
82.5
OPA681
+5V
OPA681
75
600
V
de-coupling not shown
R
RG-59
V
O
F
I
Cable
Power Supply
= 3V/V (9.54dB)
DIS
50
50 Load
V
O
®