6

Optional GND Pin for TTL Compatibility

Pin 12 is an optional GND reference used to ensure the TTL

compatibility of the DISABLE inputs. With symmetrical

supplies the GND pin may be unconnected, 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. With an external GND, the

DISABLE input is TTL compatible regardless of supply voltage

utilized.

PC Board Layout

The HFA1245’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 (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 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 HA5022

evaluation board discussed below.

Driving Capacitive Loads

Capacitive loads, such as an A/D input, or an improperly

terminated transmission line 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 2 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

bandwidth is obtained without sacrificing stability. Even so,

bandwidth still decreases as the load capacitance

the overall bandwidth is 185MHz, but the bandwidth drops

Evaluation Board

Evaluate the HFA1245’s performance using the HA5022

evaluation board (part number HA5022EVAL). Please

contact your local sales office for ordering information. The

feedback and gain setting resistors must be replaced with

the appropriate value (see “Optimum Feedback Resistor”

The modified schematic of the board is shown in Figure 3.

0

100

200

300

400

0

10

20

30

40

50

LOAD CAPACITANCE (pF)

150

250

350

50

FIGURE 2. RECOMMENDED SERIES OUTPUT RESISTOR vs

LOAD CAPACITANCE

+5V

10

μF

0.1

μF

50

Ω

GND

GND

750

Ω

750

Ω

-5V

0.1

μF

10

μF

50

Ω

IN1

OUT1

1

2

3

4

5

6

7

14

13

12

11

10

9

8

+

-

+

-

NC

NC

NC

50

Ω

OUT2

50

Ω

IN2

DIS1

DIS2

750

Ω

750

Ω

CH1

CH2

FIGURE 3. EVALUATION BOARD SCHEMATIC MODIFIED

HFA1245