OPA2680 Datasheet, PDF (13/21 Pages) Burr-Brown (TI) – Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable

transimpedance frequency response, the pole in each feed-
back network should be set to:
1/2πR_FC_F = √GBP/4πR_FC_D
which will give a corner frequency f_–3dB of approximately:
f_–3dB = √GBP/(2πR_FC_D)
High Speed
DAC
I_O
50Ω
1/2
OPA2680
R_F1
C_F1
C_D1
R_F2
V_O = I_O R_F
I_O
C_D2
C_F2
1/2
OPA2680
V_O = I_O R_F
50Ω
GBP → Gain Bandwidth
Product (Hz) for the OPA2680
FIGURE 5. High Speed DAC—Differential Transimpedance
Amplifier
WIDEBAND VIDEO MULTIPLEXING
One common application for video speed amplifiers which
include a disable pin is to wire multiple amplifier outputs
together, then select which one of several possible video
inputs to source onto a single line. This simple “Wired-OR
Video Multiplexer” can be easily implemented using the
OP2680N (SO-14 package only) as shown in Figure 6.
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 OPA2680N ensures that there is always
one amplifier controlling the line when using a wired-OR
circuit like that shown in Figure 6. 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 (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 have
been slightly increased to get a signal gain of +1 at the
matched load and provide a 75Ω output impedance to the
cable. The video multiplexer connection (Figure 6) also
insures 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.
The section on Disable Operation shows 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 (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 switch-
ing glitches for two 0V inputs drop to <20mV.
V_DIS
+5V
2kΩ
+5V
Video 1
50Ω
75Ω
DISA
1/2
OPA2680
340Ω
–5V
402Ω
340Ω
402Ω
+5V
Video 2
50Ω
75Ω
2kΩ
1/2
OPA2680
DISB
–5V
FIGURE 6. Two-Channel Video Multiplexer (SO-14 package only).
13
82.5Ω
75Ω Cable
RG-59
75Ω Load
82.5Ω
®
OPA2680