OPA3695_14 Datasheet, PDF (16/39 Pages) Texas Instruments – Triple, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3695
SBOS355A – APRIL 2008 – REVISED SEPTEMBER 2008 ............................................................................................................................................... www.ti.com
HIGH-SPEED INSTRUMENTATION
AMPLIFIER
Figure 41 shows an instrumentation amplifier circuit
based on the OPA3695. Because all three amplifiers
are on the same silicon die, the offset matching
between inputs makes this configuration an attractive
input stage for this application. The
differential-to-single-ended gain for this circuit is
2V/V. The inputs are high-impedance, with only 1.2pF
to ground at each input. The loads on the OPA3695
outputs are equal for the best harmonic distortion
possible.
V₁
1/3
OPA3695
604W
604W
604W
604W
301W
301W
1/3
OPA3695
V_OUT
604W
604W
1/3
OPA3695
V₂
Figure 41. High-Speed Instrumentation Amplifier
MULTIPLEXED CONVERTER DRIVER
The converter driver in Figure 42 multiplexes among
the three input signals with gains of +2V/V, +4V/V,
and +8V/V. The OPA3695 enable and disable times
support multiplexing among video signals. The
make-before-break disable characteristic of the
OPA3695 ensures that the output is always under
control. To avoid large switching glitches, it is best to
switch when the signal on the amplifier inputs are
very close to each other.
The voltage difference appearing between the
inverting node and the noninverting node should not
exceed ±1.2V. This difference can occur when the
individual amplifier is disabled and a voltage is
applied at the summing node of the three amplifiers.
The resulting inverting node voltage of the disabled
amplifier is easily calculated by using simple resistor
voltage divider methods. In general, as the gain of the
amplifier increases, the less impact this issue has on
the system because of the increased R_F/R_G ratio.
The output resistors isolate the outputs from each
other when switching between channels. The
feedback network of the disabled channels forms part
of the load seen by the enabled amplifier, attenuating
the signal slightly.
V₁
1/3
OPA3695
R_G
604W
R_F
604W
100W
4.99kW
0.1mF
4.99kW
0.1mF
+5V
V₂
1/3
OPA3695
R_G
169W
R_F
511W
V₃
1/3
OPA3695
100W
0.1mF
100pF
100W
0.1mF
REFT REFB
+3.5V +1.5V
+In
-In
CM
ADS828
10-Bit
75MSPS
R_G
57.6W
R_F
402W
Selection
Logic
Figure 42. Multiplexed Converter Driver
16
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