OPA2890_08 Datasheet, PDF (19/35 Pages) Texas Instruments – Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2890
www.ti.com.................................................................................................................................................... SBOS364B – DECEMBER 2007 – REVISED MAY 2008
Typically, channel switching is performed either on
sync or retrace time in the video signal. The two
inputs are approximately equal at this point. The
make-before-break disable characteristic of the
OPA2890 ensures that there is always one amplifier
controlling the line when using a wired-OR circuit
such as that shown in Figure 54. Because both inputs
may be on for a short period during the transition
between channels, the outputs are combined through
the output impedance matching resistors (82.5Ω 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 gain and output matching resistor are
slightly increased to get a signal gain of +1V/V at the
matched load and provide a 75Ω output impedance
to the cable. The video multiplexer connection (see
Figure 54) also ensures that the maximum differential
voltage across the inputs of the unselected channel
does not exceed the rated ±1.2V maximum for
standard video signal levels.
See the Disable Operation section for the turn-on and
turn-off switching glitches using a 0V input for a
single channel is typically less than ±50mV. Where
two outputs are switched (see Figure 54), the output
line is always under the control of one amplifier or the
other as a result of the make-before-break disable
timing. In this case, the switching glitches for two 0V
inputs drops to less than 20mV.
HIGH-SPEED DELAY CIRCUIT
The OPA2890 makes an ideal amplifier for a variety
of active filter designs. Figure 55 illustrates a circuit
that uses the two amplifiers within the dual OPA2890
to design a two-stage analog delay circuit. For
simplicity, the circuit uses a dual-supply (±5V)
operation, but it can also be modified to operate on a
signal supply. The input to the first filter stage is
driven by the OPA890 as a gain of +2V/V to isolate
the signal input from the filter network.
Each of the two filter stages is a 1st-order filter with a
voltage gain of +1V/V. The delay time through one
filter is given by Equation 2.
t_GR0 = 2RC
(2)
For a more accurate analysis of the circuit, consider
the group delay for the amplifiers. For example, in the
case of the OPA2890, the group delay in the
bandwidth from 1MHz to 100MHz is approximately
1.0ns. To account for this delay, modify the transfer
function, which now comes out to be:
t_GR = 2 (2RC + T_D)
(3)
with T_D = (1/360) × (dφ/df) = delay of the op amp
itself. The values of resistors R_F and R_G should be
equal and low to avoid parasitic effects. If the all-pass
filter is designed for very low delay times, include
parasitic board capacitances to calculate the correct
delay time. Simulating this application using the
PSpice model of the OPA2890 allows this design to
be tuned to the desired performance.
V_IN
OPA890
750W
750W
C
R
1/2
OPA2890
R_G
402W
R_F
402W
C
R
R_G
402W
1/2
OPA2890
R_F
402W
V_OUT
Figure 55. Two-Stage, All-Pass Network
Copyright © 2007–2008, Texas Instruments Incorporated
Product Folder Link(s): OPA2890
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