4

Application Information

Closed Loop Gain Selection

The HFA1112 features a novel design which allows the user

to select from three closed loop gains, without any external

components. The result is a more flexible product, fewer part

types in inventory, and more efficient use of board space.

This “buffer” operates in closed loop gains of -1, +1, or +2, and

gain selection is accomplished via connections to the

±inputs.

Applying the input signal to +IN and floating -IN selects a gain

of +1, while grounding -IN selects a gain of +2. A gain of -1 is

obtained by applying the input signal to -IN with +IN grounded.

The table below summarizes these connections:

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 (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.

For unity gain applications, care must also be taken to

minimize the capacitance to ground seen by the amplifier’s

inverting input. At higher frequencies this capacitance will

tend to short the -INPUT to GND, resulting in a closed loop

gain which increases with frequency. This will cause

excessive high frequency peaking and potentially other

problems as well.

An example of a good high frequency layout is the

Evaluation 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

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.

limiting system bandwidth well below the amplifier

Rise Time

25

-

0.82

-

ns

25

-

1.06

-

ns

25

-

1.00

-

ns

Overshoot

Notes 2, 3, 4)

25

-

12

30

%

25

-

45

65

%

25

-

6

20

%

0.1% Settling Time (Note 3)

25

-

11

-

ns

0.05% Settling Time

25

-

15

-

ns

Overdrive Recovery Time

25

-

8.5

-

ns

Differential Gain

25

-

0.03

-

%

25

-

0.02

-

%

Differential Phase

25

-

0.05

-

Degrees

25

-

0.04

-

Degrees

NOTES:

2. This parameter is not tested. The limits are guaranteed based on lab characterization, and reflect lot-to-lot variation.

3. See Typical Performance Curves for more information.

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX

UNITS

GAIN

CONNECTIONS

+INPUT (PIN 3)

-INPUT (PIN 2)

-1

GND

Input

+1

Input

NC (Floating)

+2

Input

GND

HFA1112