HS-1245RH 데이터 시트보기 (PDF) - Intersil

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HS-1245RH

Radiation Hardened, Dual, High Speed, Low Power Video Operational Amplifier with Output Disable

Intersil 

HS-1245RH

Application Information

Optimum Feedback Resistor

Although a current feedback amplifier’s 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-1245RH design is

optimized for a 560Ω RF at a gain of +2. Decreasing RF

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

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. 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.

GAIN

(ACL)

-1

+1

+2

RF (Ω)

510

560 (+RS = 560Ω)

560

BANDWIDTH

(MHz)

230

290

530

Non-Inverting Input Source Impedance

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.

Optional GND Pin for TTL Compatibility

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.

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 290MHz (for AV = +1). 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 = 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

AV = +2

10

0

0 50 100 150 200 250 300 350 400

LOAD CAPACITANCE (pF)

FIGURE 1. RECOMMENDED SERIES OUTPUT RESISTOR vs

LOAD CAPACITANCE

2

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