AD8002AR-EBZ(2015) Просмотр технического описания (PDF) - Analog Devices
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AD8002AR-EBZ Datasheet PDF : 21 Pages
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Data Sheet
The current feedback nature of the op amps, in addition to
enabling the wide bandwidth, provides an output drive of
more than 3 V p-p into a 20 Ω load for each output at 20 MHz.
Conversely, the voltage feedback nature provides symmetrical
high impedance inputs and allows the use of reactive
components in the feedback network.
The circuit consists of the two op amps, each configured as a
unity-gain follower by the 511 Ω RA feedback resistors between
the output and inverting input of each op amp. The output of
each op amp has a 511 Ω RB resistor to the inverting input of
the other op amp. Thus, each output drives the other op amp
through a unity-gain inverter configuration. By connecting
the two amplifiers as cross-coupled inverters, the outputs of the
amplifiers are freed to be equal and opposite, assuring zero
output common-mode voltage.
Using this circuit configuration, the common-mode signal of
the outputs is reduced. If one output increases slightly, the
negative input to the other op amp drives its output slightly
lower and thus preserves the symmetry of the complementary
outputs, which reduces the common-mode signal. The common-
mode output signal was measured as −50 dB at 1 MHz.
Looking at this configuration overall, there are two high
impedance inputs (the +IN1, +IN2 of each op amp), two
low impedance outputs, and a high open-loop gain. The two
noninverting inputs and the output of the Op Amp 2 structure
looks like a voltage feedback op amp having two symmetrical,
high impedance inputs and one output. The +IN1, +IN2 to
Op Amp 2 is the noninverting input (it has the same polarity as
OUT2) and the +IN1, +IN2 to Op Amp 1 is the inverting input
(opposite polarity of Output 2).
With a feedback resistor, RF, an input resistor, RG, and the
grounding of the +IN1, +IN2 of Op Amp 2, a feedback
amplifier is formed. This configuration is similar to a voltage
feedback amplifier in an inverting configuration if only OUT2
is considered. The addition of OUT1 makes the amplifier a
differential output.
The differential gain of this circuit is
G
=
RF
RG
× 1 +
RA
RB
where:
RF/RG is the gain of the overall op amp configuration and is the
same as for an inverting op amp except for the polarity. If OUT1
is used as the output reference, the gain is positive.
1 + RA/RB is the noise gain of each individual op amp in its
noninverting configuration.
AD8002
The resulting architecture offers several advantages. First, the
gain can be changed by changing a single resistor. Changing
either RF or RG changes the gain as in an inverting op amp
circuit. For most types of differential circuits, more than one
resistor must be changed to change gain and still maintain good
common-mode rejection (CMR).
Reactive elements can be used in the feedback network. This is
in contrast to current feedback amplifiers that restrict the use of
reactive elements in the feedback op amp. The circuit described
requires about 0.9 pF of capacitance in shunt across RF to optimize
peaking and realize a −3 dB bandwidth of more than 200 MHz.
The peaking exhibited by the circuit is very sensitive to the
value of this capacitor. Parasitics in the board layout on the
order of tenths of picofarads influences the frequency response
and the value required for the feedback capacitor, thus a good
layout is essential.
The shunt capacitor type selection is also critical. A good
microwave type chip capacitor with high Q was found to yield
best performance. The device selected for this circuit was a
Murata Erie MA280R9B.
The distortion was measured at 20 MHz with a 3 V p-p input
and a 100 Ω load on each output. For OUT1, the distortion is
−37 dBc and −41 dBc for the second and third harmonics,
respectively. For OUT2, the second harmonic is −35 dBc and
the third harmonic is −43 dBc.
6
CC = 0.9pF
4
2
0
–2
–4
–6
OUT+
–8
–10
OUT–
–12
–14
1M
10M
100M
1G
FREQUENCY (Hz)
Figure 56. Differential Driver Frequency Response
Rev. E | Page 17 of 21
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