AD8036_15 Datasheet, PDF (16/24 Pages) Analog Devices – Low Distortion, Wide Bandwidth Voltage Feedback Clamp Amps

AD8036/AD8037
THEORY OF OPERATION
General
The AD8036 and AD8037 are wide bandwidth, voltage feedback
clamp amplifiers. Since their open-loop frequency response fol-
lows the conventional 6 dB/octave roll-off, their gain bandwidth
product is basically constant. Increasing their closed-loop gain
results in a corresponding decrease in small signal bandwidth. This
can be observed by noting the bandwidth specification, between
the AD8036 (gain of 1) and AD8037 (gain of 2). The AD8036/
AD8037 typically maintain 65 degrees of phase margin. This
high margin minimizes the effects of signal and noise peaking.
While the AD8036 and AD8037 can be used in either an invert-
ing or noninverting configuration, the clamp function will only
work in the noninverting mode. As such, this section shows con-
nections only in the noninverting configuration. Applications
that require an inverting configuration will be discussed in the
Applications section. In applications that do not require clamp-
ing, Pins 5 and 8 (respectively V_L and V_H) may be left floating.
See Input Clamp Amp Operation and Applications sections
otherwise.
Feedback Resistor Choice
The value of the feedback resistor is critical for optimum perfor-
mance on the AD8036 (gain +1) and less critical as the gain
increases. Therefore, this section is specifically targeted at
the AD8036.
At minimum stable gain (+1), the AD8036 provides optimum
dynamic performance with R_F = 140 Ω. This resistor acts only
as a parasitic suppressor against damped RF oscillations that
can occur due to lead (input, feedback) inductance and parasitic
capacitance. This value of R_F provides the best combination of
wide bandwidth, low parasitic peaking, and fast settling time.
In fact, for the same reasons, a 100–130 Ω resistor should be
placed in series with the positive input for other AD8036 non-
inverting configurations. The correct connection is shown in
Figure 3.
G
=
1+
R_F
R_G
+V_S
10␮F
V_H
0.1␮F
100 - 130⍀
V_IN
R_TERM
AD8036/
AD8037
0.1␮F
V_L
10␮F
–V_S
R_G
V_OUT
R_F
Figure 3. Noninverting Operation
For general voltage gain applications, the amplifier bandwidth
can be closely estimated as:
f _{3 dB}
≅
2π
ω_O

¹

+


R_F
R_G




This estimation loses accuracy for gains of +2/–1 or lower due
to the amplifier’s damping factor. For these “low gain” cases,
the bandwidth will actually extend beyond the calculated value
(see Closed-Loop BW plots, TPCs 13 and 25).
Pulse Response
Unlike a traditional voltage feedback amplifier, where the slew
speed is dictated by its front end dc quiescent current and gain
bandwidth product, the AD8036 and AD8037 provide “on
demand” current that increases proportionally to the input
“step” signal amplitude. This results in slew rates (1200 V/µs)
comparable to wideband current feedback designs. This, com-
bined with relatively low input noise current (2.1 pA/√Hz), gives
the AD8036 and AD8037 the best attributes of both voltage and
current feedback amplifiers.
Large Signal Performance
The outstanding large signal operation of the AD8036 and
AD8037 is due to a unique, proprietary design architecture.
In order to maintain this level of performance, the maximum
350 V-MHz product must be observed, (e.g., @ 100 MHz,
V_O ≤ 3.5 V p-p).
Power Supply and Input Clamp Bypassing
Adequate power supply bypassing can be critical when optimiz-
ing the performance of a high frequency circuit. Inductance in
the power supply leads can form resonant circuits that produce
peaking in the amplifier’s response. In addition, if large current
transients must be delivered to the load, then bypass capacitors
(typically greater than 1 µF) will be required to provide the best
settling time and lowest distortion. A parallel combination of at
least 4.7 µF, and between 0.1 µF and 0.01 µF, is recommended.
Some brands of electrolytic capacitors will require a small series
damping resistor ≈4.7 Ω for optimum results.
When the AD8036 and AD8037 are used in clamping mode,
and a dc voltage is connected to clamp inputs V_H and V_L, a 0.1 µF
bypassing capacitor is required between each input pin and
ground in order to maintain stability.
Driving Capacitive Loads
The AD8036 and AD8037 were designed primarily to drive
nonreactive loads. If driving loads with a capacitive compo-
nent is desired, the best frequency response is obtained by
the addition of a small series resistance as shown in Figure 4.
The accompanying graph shows the optimum value for R_SERIES
vs. capacitive load. It is worth noting that the frequency response
of the circuit when driving large capacitive loads will be domi-
nated by the passive roll-off of R_SERIES and C_L. For capacitive
loads of 6 pF or less, no R_SERIES is necessary.
R_F
R_IN
R_IN
AD8036/
AD8037
R_SERIES
R_L
1k⍀
C_L
Figure 4. Driving Capacitive Loads
REV. B
–15–