HS-1245RH
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.
Application Information
Optimum Feedback Resistor
Although a current feedback amplifier’s bandwidth
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.
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 R .
All current feedback amplifiers require a feedback resistor,
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.
F
even for unity gain applications, and R , in conjunction with
F
the internal compensation capacitor, sets the dominant pole
of the frequency response. Thus, the amplifier’s bandwidth is
inversely proportional to R . The HS-1245RH design is
F
optimized for a 560Ω R at a gain of +2. Decreasing R
F
F
decreases stability, resulting in excessive peaking and
overshoot (Note: Capacitive feedback will cause the same
problems due to the feedback impedance decrease at higher
frequencies). At higher gains the amplifier is more stable, so
Driving Capacitive Loads
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
R can be decreased in a trade-off of stability for bandwidth.
F
The table below lists recommended R values for various
F
avoided by placing a resistor (R ) in series with the output
S
gains, and the expected bandwidth. For good channel-to-
channel gain matching, it is recommended that all resistors
(termination as well as gain setting) be ±1% tolerance or better.
Note that a series input resistor, on +IN, is required for a gain of
+1, to reduce gain peaking and increase stability.
prior to the capacitance.
Figure 1 details starting points for the selection of this
resistor. The points on the curve indicate the R and C
combinations for the optimum bandwidth, stability, and
S
L
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.
GAIN
(A
BANDWIDTH
(MHz)
)
R (Ω)
F
CL
-1
+1
+2
510
230
290
530
560 (+R = 560Ω)
S
R
and C form a low pass network at the output, thus
L
S
560
limiting system bandwidth well below the amplifier
bandwidth of 290MHz (for A = +1). By decreasing R as
Non-Inverting Input Source Impedance
V
S
C increases (as illustrated in the curves), the maximum
L
For best operation, the D.C. source impedance looking out of
the non-inverting input should be ≥50Ω. This is especially
important in inverting gain configurations where the non-
inverting input would normally be connected directly to GND.
bandwidth is obtained without sacrificing stability. Even so,
bandwidth does decrease as you move to the right along
the curve. For example, at A = +1, R = 62Ω, C = 40pF,
V
S
L
the overall bandwidth is limited to 180MHz, and bandwidth
drops to 70MHz at A = +1, R = 8Ω, C = 400pF.
Optional GND Pin for TTL Compatibility
V
S
L
50
The HS-1245RH derives an internal GND reference for the
digital circuitry as long as the power supplies are
symmetrical about GND. The GND reference is used to
ensure the TTL compatibility of the DISABLE inputs. With
symmetrical supplies the GND pin (Pin 12) may be floated,
or connected directly to GND. If asymmetrical supplies (e.g.
+10V, 0V) are utilized, and TTL compatibility is desired, the
GND pin must be connected to GND.
40
30
20
10
0
A
= +1
V
A
= +2
150
V
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
resistors and chip capacitors is strongly recommended,
while a solid ground plane is a must!
0
50
100
200
250
300
350
400
LOAD CAPACITANCE (pF)
FIGURE 1. RECOMMENDED SERIES OUTPUT RESISTOR vs
LOAD CAPACITANCE
2
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