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HFA1245 Datasheet(PDF) 6 Page - Intersil Corporation

Part No. HFA1245
Description  Dual, 420MHz, Low Power, Video, Current Feedback Operational Amplifier with Disable
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
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HFA1245 Datasheet(HTML) 6 Page - Intersil Corporation

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6
Optional GND Pin for TTL Compatibility
Pin 12 is an optional GND reference used to ensure the TTL
compatibility of the DISABLE inputs. With symmetrical
supplies the GND pin may be unconnected, 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. With an external GND, the
DISABLE input is TTL compatible regardless of supply voltage
utilized.
PC Board Layout
The HFA1245’s frequency response depends greatly on the
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!
Attention should be given to decoupling the power supplies.
A large value (10
μF) tantalum in parallel with a small value
(0.1
μF) 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 at the amplifier’s inverting input (-IN), as this
capacitance causes gain peaking, pulse overshoot, and if
large enough, instability. To reduce this capacitance, the
designer should remove the ground plane under traces
connected to -IN, and keep connections to -IN as short as
possible.
An example of a good high frequency layout is the HA5022
evaluation board discussed below.
Driving Capacitive Loads
Capacitive loads, such as an A/D input, or an improperly
terminated transmission line 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 2 details starting points for the selection of this
resistor. The points on the curve indicate the RS and CL
combinations 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
bandwidth of 260MHz (for AV = +1). By decreasing RS as
CLincreases (as illustrated in the curves), the maximum
bandwidth is obtained without sacrificing stability. Even so,
bandwidth still decreases as the load capacitance
increases. For example, at AV = +1, RS = 45Ω, CL = 40pF,
the overall bandwidth is 185MHz, but the bandwidth drops
to 85MHz at AV = +1, RS = 9Ω, CL = 330pF.
Evaluation Board
Evaluate the HFA1245’s performance using the HA5022
evaluation board (part number HA5022EVAL). Please
contact your local sales office for ordering information. The
feedback and gain setting resistors must be replaced with
the appropriate value (see “Optimum Feedback Resistor”
table) for the gain being evaluated. Also, replace the two 0Ω
series output resistors (RS) with 50Ω resistors.
The modified schematic of the board is shown in Figure 3.
0
100
200
300
400
0
10
20
30
40
50
LOAD CAPACITANCE (pF)
AV = +1
AV = +2
150
250
350
50
FIGURE 2. RECOMMENDED SERIES OUTPUT RESISTOR vs
LOAD CAPACITANCE
+5V
10
μF
0.1
μF
50
Ω
GND
GND
750
Ω
750
Ω
-5V
0.1
μF
10
μF
50
Ω
IN1
OUT1
1
2
3
4
5
6
7
14
13
12
11
10
9
8
+
-
+
-
NC
NC
NC
50
Ω
OUT2
50
Ω
IN2
DIS1
DIS2
750
Ω
750
Ω
CH1
CH2
RS
RS
FIGURE 3. EVALUATION BOARD SCHEMATIC MODIFIED
FOR AV = +2
HFA1245


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