HS-1145RH(1999) データシートの表示（PDF） - Intersil

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HS-1145RH
(Rev.:1999)

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

Intersil 

HS-1145RH

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-1145RH design is

optimized for RF = 510Ω 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, however, 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 a gain of +1, a

resistor (+RS) in series with +IN is required to reduce gain

peaking and increase stability.

GAIN

(ACL)

-1

+1

+2

+5

+10

RF (Ω)

425

510 (+RS = 510Ω)

510

200

180

BANDWIDTH

(MHz)

300

270

330

300

130

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.

DISABLE Input TTL Compatibility

The HS-1145RH derives an internal GND reference for the

digital circuitry as long as the power supplies are symmetrical

about GND. With symmetrical supplies the digital switching

threshold (VTH = (VIH + VIL)/2 = (2.0 + 0.8)/2) is 1.4V, which

ensures the TTL compatibility of the DISABLE input. If

asymmetrical supplies (e.g., +10V, 0V) are utilized, the

switching threshold becomes:

VTH = V-----+----2-+----V------ + 1.4V

and the VIH and VIL levels will be VTH ± 0.6V, respectively.

Optional GND Pad (Die Use Only) for TTL

Compatibility

The die version of the HS-1145RH provides the user with a

GND pad for setting the disable circuitry GND reference.

With symmetrical supplies the GND pad may be left

unconnected, or tied directly to GND. If asymmetrical

supplies (e.g., +10V, 0V) are utilized, and TTL compatibility

is desired, die users must connect the GND pad to GND.

With an external GND, the DISABLE input is TTL compatible

regardless of supply voltage utilized.

Pulse Undershoot and Asymmetrical Slew Rates

The HS-1145RH utilizes a quasi-complementary output

stage to achieve high output current while minimizing

quiescent supply current. In this approach, a composite

device replaces the traditional PNP pulldown transistor. The

composite device switches modes after crossing 0V,

resulting in added distortion for signals swinging below

ground, and an increased undershoot on the negative

portion of the output waveform (See Figures 5, 8, and 11).

This undershoot isn’t present for small bipolar signals, or

large positive signals. Another artifact of the composite

device is asymmetrical slew rates for output signals with a

negative voltage component. The slew rate degrades as the

output signal crosses through 0V (See Figures 5, 8, and 11),

resulting in a slower overall negative slew rate. Positive only

signals have symmetrical slew rates as illustrated in the

large signal positive pulse response graphs (See Figures 4,

7, and 10).

PC Board Layout

This amplifier’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 (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

device’s input and output connections. Capacitance,

parasitic or planned, connected to 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

Evaluation Board shown in Figure 2.

2

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