HFA1205(1998) Просмотр технического описания (PDF) - Intersil

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HFA1205
(Rev.:1998)

Dual, 400MHz, Low Power, Video Operational Amplifier

Intersil 

HFA1205

Non-inverting Input Source Impedance

For best operation, the DC source impedance seen by 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.

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!

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.

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

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

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 280MHz (for AV = +1). By decreasing RS as

CL increases (as illustrated in the curves), the maximum

bandwidth is obtained without sacrificing stability. In spite

of this, bandwidth decreases as the load capacitance

increases. For example, at AV = +1, RS = 62Ω, CL = 40pF,

the overall bandwidth is limited to 180MHz, and bandwidth

drops to 70MHz at AV = +1, RS = 8Ω, CL = 400pF.

50

40

30

20

AV = +1

10

AV = +2

0

0 50 100 150 200 250 300 350 400

LOAD CAPACITANCE (pF)

FIGURE 1. RECOMMENDED SERIES OUTPUT RESISTOR vs

LOAD CAPACITANCE

Evaluation Board

The performance of the HFA1205 may be evaluated using

the HA5023 Evaluation Board. The feedback and gain

setting resistors must be replaced with the appropriate value

(see “Optimum Feedback Resistor” section) for the gain

being evaluated. Also, replace the two 0Ω series output

resistors with 50Ω resistors.

To order evaluation boards (Part Number HA5023EVAL),

please contact your local sales office.

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