OPA692 Datasheet, PDF (15/30 Pages) Burr-Brown (TI) – Wideband, Fixed Gain Video BUFFER AMPLIFIER With Disable

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 OPA692 ensures that there is always
one amplifier controlling the line when using a wired-OR
circuit (see 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 resis-
tors (68.1Ω in this case). When one channel 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 (see Figure 4) also insures
that the maximum differential voltage across the inputs of the
unselected channel do not exceed the rated ±1.2V maximum
for standard video signal levels. In any case, V_OUT must be
< ±2.6V_PP in order to not exceed the absolute maximum
differential input voltage (±1.2V) on the disabled channel.
The Disable Operation section shows the turn-on and turn-off
switching glitches using a grounded input for a single chan-
nel is typically less than ±50mV. Where two outputs are
switched (see 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 drops to < 20mV.
4-CHANNEL FREQUENCY CHANNELIZER
The circuit of Figure 5 is a 4-channel multiplexer. In this
circuit the OPA691 provides the drive for all four channels.
Each channel includes a bandpass filter and each bandpass
filter is set for a different frequency band. This allows the
channelizing part of this circuit. The role of the OPA692 is to
provide impedance isolation. This is done through the use of
four matching resistances (59Ω in this case). These match-
ing resistors ensure that the signals will combine during the
transition between channels. They have been used to get a
gain of +1 at the load.
This circuit may be used with a different number of channels.
Its limitation comes from the drive requirement for each
channel, as well as the minimum acceptable return loss.
The output resistor value (R_O) to keep a gain of +1 at the
load, depends on the number of channels. For the OPA692,
Equation 1 gives:
(1)
[ ] _{[ ] RO =}
75Ω • (n – 2) + 804Ω
2

• _{}
1+
241200Ω
75Ω • (n − 2) + 804Ω ²

– 1_{}
Where n = number of devices in multiplexer.
75Ω
#1
75Ω
#2
+5V
OPA691
75Ω
–5V
#3
75Ω
#4
FIGURE 5. 4-Channel Frequency Channelizer.
+5V
DIS 1
R_O
59Ω
OPA692
75Ω
–5V
+5V
DIS 2
R_O
59Ω
OPA692
75Ω
–5V
+5V
DIS 3
R_O
59Ω
OPA692
75Ω
–5V
+5V
DIS 4
R_O
59Ω
OPA692
75Ω
–5V
G = +2 Stages
75Ω Cable
RG-59
V_OUT
75Ω Load
OPA692
15
SBOS236E
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