OPA2889 Datasheet, PDF (19/35 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
www.ti.com
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
OPA2889 ensures that there is always one amplifier
controlling the line when using a wired-OR circuit like
that shown in Figure 55. 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 55) 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 55), 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 < 20mV.
SBOS373 – JUNE 2007
HIGH-SPEED DELAY CIRCUIT
The OPA2889 makes an ideal amplifier for a variety
of active filter designs. Figure 56 illustrates a circuit
that uses the two amplifiers within the dual OPA2889
to design a 2-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 3.
t_GR0 = 2RC
(3)
For a more accurate analysis of the circuit, consider
the group delay for the amplifiers. For example, in
the case of the OPA2889, 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)
(4)
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 OPA2889 allows this
design to be tuned to the desired performance.
V_IN
OPA890
750W
750W
C
R
1/2
OPA2889
R_G
402W
R_F
402W
C
R
R_G
402W
Figure 56. 2-Stage, All-Pass Network
1/2
OPA2889
R_F
402W
V_OUT
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