OPA682N BURR-BROWN [Burr-Brown Corporation], OPA682N Datasheet - Page 15

• Manufacturer Part Number: OPA682N
• Description: Wideband, Fixed Gain BUFFER AMPLIFIER With Disable
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

Typically, channel switching is performed either on sync or
retrace time in the video signal. The two inputs are approxi-
mately equal at this time. The “make-before-break” disable
characteristic of the OPA682 ensures that there is always
one amplifier controlling the line when using a wired-OR
circuit like that shown in Figure 4. Since both inputs may be
on for a short period during the transition between channels,
the outputs are combined through the output impedance
matching resistors (68.1
is disabled, its feedback network forms part of the output
impedance and slightly attenuates the signal in getting out
onto the cable. The matching resistors have been set to get
a signal gain of +1 at the load while providing > 20dB return
loss at the load.
The video multiplexer connection (Figure 4) also insures
that the maximum differential voltage across the inputs of
the unselected channel do not exceed the rated
maximum for standard video signal levels. In any case,
V
maximum differential input voltage ( 1.2V) on the disabled
part.
The section on Disable Operation shows the turn-on and
turn-off switching glitches using a grounded input for a
FIGURE 5. Butterworth LP Filter with Delay Equalization.
OUT
must be < 2.6Vp-p in order to not exceed the absolute
V
IN
100pF
115
in this case). When one channel
400
OPA682
400
220pF
49.9
1.2V
15
105
95.3
56pF
68pF
single channel is typically less than 50mV. Where two
outputs are switched (as shown in Figure 4), 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.
DELAY-EQUALIZED LOWPASS FILTER
The circuit in Figure 5 realizes a 5th-order Butterworth
lowpass filter with a –3dB bandwidth of 20MHz and group
delay equalization. This filter is based on the KRC active
filter topology using amplifiers with a fixed positive gain 1.
The OPA682 makes a good amplifier for this type of filter.
The first stage is the group delay equalizer, which is based
on a gain of –1. The second stage has a high-Q pole, and uses
a gain of +2 for minimum component sensitivity. The second
stage also produces a real pole. The last stage has a low-Q
pole, and uses a gain of +1 for minimum component sensi-
tivity.
The component values have been pre-distorted to compensate
for the op amps’s parasitic effects. The low-Q pole section
was placed last to minimize noise peaking in the passband,
while maintaining good dynamic range performance.
226
226
27pF
39pF
(Open)
400
400
OPA682
OPA682
400
400
OPA682
V
OUT
®