2

Application Information

Optimum Feedback Resistor

The enclosed plots of inverting and non-inverting frequency

response illustrate the performance of the HS-1120RH in

various gains. Although the 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

feedback amplifiers require a feedback resistor, even for

internal compensation capacitor, sets the dominant pole of

the frequency response. Thus, the amplifier’s bandwidth is

optimized for a 510

unity gain application decreases stability, resulting in

excessive peaking and overshoot. At higher gains the

off of stability for bandwidth.

gains, and the expected bandwidth.

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 resis-

tors 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 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.

An example of a good high frequency layout is the Evalua-

tion Board shown in Figure 2.

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

prior to the capacitance.

Figure 1 details starting points for the selection of this resis-

tions 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.

limiting system bandwidth well below the amplifier band-

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.

bandwidth is limited to 300MHz, and bandwidth drops to

Evaluation Board

The performance of the HS-1120RH may be evaluated using

the HFA11XXEVAL Evaluation Board.

The layout and schematic of the board are shown in

Figure 2. To order evaluation boards, please contact your

local sales office.

Offset Adjustment

The output offset voltage of the HS-1120RH may be nulled via

connections to the BAL pins. Unlike a voltage feedback

amplifier, offset adjustment is accomplished by varying the sign

With voltage feedback amplifiers, bias currents are matched

and bias current induced offset errors are nulled by matching

the impedances seen at the positive and negative inputs. Bias

GAIN

BANDWIDTH

(MHz)

-1

430

580

+1

510

850

+2

360

670

+5

150

520

+10

180

240

+19

270

125

LOAD CAPACITANCE (pF)

50

45

40

35

30

25

20

15

10

5

0

0

40

80

120

160

200

240

280

320

360

400

FIGURE 1. RECOMMENDED SERIES OUTPUT RESISTOR vs

LOAD CAPACITANCE

HS-1120RH