HS-1120RH データシートの表示（PDF） - Intersil
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HS-1120RH Datasheet PDF : 11 Pages
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HS-1120RH
Application Information
Driving Capacitive Loads
Optimum Feedback Resistor
The enclosed plots of inverting and non-inverting frequency
response illustrate the performance of the HS-1120RH in
various gains. Although the bandwidth dependency on
closed loop gain isn’t as severe as that of a voltage feedback
amplifier, there can be an appreciable decrease in
bandwidth at higher gains. This decrease may be minimized
by taking advantage of the current feedback amplifier’s
unique relationship between bandwidth and RF. All current
feedback amplifiers require a feedback resistor, even for
unity gain applications, and RF, in conjunction with the
internal compensation capacitor, sets the dominant pole of
the frequency response. Thus, the amplifier’s bandwidth is
inversely proportional to RF. The HS-1120RH design is
optimized for a 510Ω RF at a gain of +1. Decreasing RF in a
unity gain application decreases stability, resulting in
excessive peaking and overshoot. At higher gains the
amplifier is more stable, so RF can be decreased in a trade-
off of stability for bandwidth.
The table below lists recommended RF values for various
gains, and the expected bandwidth.
GAIN
(ACL)
-1
+1
+2
+5
+10
+19
RF (Ω)
430
510
360
150
180
270
BANDWIDTH
(MHz)
580
850
670
520
240
125
PC Board Layout
The frequency response of this amplifier depends greatly on
the amount of care taken in designing the PC board. The
use of low inductance components such as chip resis-
tors and chip capacitors is strongly recommended,
while a solid ground plane is a must!
Capacitive loads, such as an A/D input, or an improperly
terminated transmission line will degrade the amplifier’s
phase margin resulting in frequency response peaking and
possible oscillations. In most cases, the oscillation can be
avoided by placing a resistor (RS) in series with the output
prior to the capacitance.
Figure 1 details starting points for the selection of this resis-
tor. The points on the curve indicate the RS and CL combina-
tions for the optimum bandwidth, stability, and settling time,
but experimental fine tuning is recommended. Picking a
point above or to the right of the curve yields an overdamped
response, while points below or left of the curve indicate
areas of underdamped performance.
RS and CL form a low pass network at the output, thus
limiting system bandwidth well below the amplifier band-
width of 850MHz. By decreasing RS as CLincreases (as
illustrated in the curves), the maximum bandwidth is
obtained without sacrificing stability. Even so, bandwidth
does decrease as you move to the right along the curve.
For example, at AV = +1, RS = 50Ω, CL = 30pF, the overall
bandwidth is limited to 300MHz, and bandwidth drops to
100MHz at AV = +1, RS = 5Ω, CL = 340pF.
50
45
40
AV = +1
35
30
25
20
15
10
5 AV = +2
0
0 40 80 120 160 200 240 280 320
LOAD CAPACITANCE (pF)
360 400
FIGURE 1. RECOMMENDED SERIES OUTPUT RESISTOR vs
LOAD CAPACITANCE
Attention should be given to decoupling the power supplies.
A large value (10µF) tantalum in parallel with a small value
(0.1µF) chip capacitor works well in most cases.
Terminated microstrip signal lines are recommended at the
input and output of the device. Capacitance directly on the
output must be minimized, or isolated as discussed in the
next section.
Care must also be taken to minimize the capacitance to
ground seen by the amplifier’s inverting input (-IN). The
larger this capacitance, the worse the gain peaking, resulting
in pulse overshoot and possible instability. To this end, it is
recommended that the ground plane be removed under
traces connected to -IN, and connections to -IN should be
kept as short as possible.
An example of a good high frequency layout is the Evalua-
tion Board shown in Figure 2.
Evaluation Board
The performance of the HS-1120RH may be evaluated using
the HFA11XXEVAL Evaluation Board.
The layout and schematic of the board are shown in
Figure 2. To order evaluation boards, please contact your
local sales office.
Offset Adjustment
The output offset voltage of the HS-1120RH may be nulled via
connections to the BAL pins. Unlike a voltage feedback
amplifier, offset adjustment is accomplished by varying the sign
and/or magnitude of the inverting input bias current (-IBIAS).
With voltage feedback amplifiers, bias currents are matched
and bias current induced offset errors are nulled by matching
the impedances seen at the positive and negative inputs. Bias
2
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