9

HFA-0005

Applications Information

Offset Adjustment

When applications require the offset voltage to be as low as

possible, the figure below shows two possible schemes for

adjusting offset voltage.

FIGURE 28. INVERTING GAIN

For a voltage follower application, use the circuit in Figure 29

10M

Ω, so the adjustment resistors will cause only a very

small gain error.

FIGURE 29. NON-INVERTING GAIN

PC Board Layout Guidelines

When designing with the HFA-0005, good high frequency

(RF) techniques should be used when making a PC board. A

massive ground plane should be used to maintain a low

impedance ground. Proper shielding and use of short

interconnection leads are also very important.

To achieve maximum high frequency performance, the use

of low impedance transmission lines with impedance

matching is recommended: 50

Ω lines are common in

communications and 75

Ω lines in video systems. Impedance

matching is important to minimize reflected energy therefore

minimizing

transmitted

signal

distortion.

This

is

accomplished by using a series matching resistor (50

Ω or

75

Ω), matched transmission line (50Ω or 75Ω), and a

matched terminating resistor, as shown in Figure 30. Note

that there will be a 6dB loss from input to output. The

HFA-0005 has an integral 50

Ω ±20% resistor connected to

the op amp’s output with the other end of the resistor pinned

out. This 50

Ω resistor can be used as the series resistor

instead of an external resistor.

50

+

50K

100K

+5V

100

-5V

Adjustment Range

V

R

2

R

1

-----





±

≅

+

50K

+V

-V

100K

100

Adjustment Range

V

R

2

R

1

-----





±

≅

Ga in

1

R

F

R

I

R

2

+

---------------





+

≅

FIGURE 30.

PC board traces can be made to look like a 50

Ω or 75Ω

transmission line, called microstrip. Microstrip is a PC board

trace with a ground plane directly beneath, on the opposite

side of the board, as shown in Figure 31.

FIGURE 31.

When manufacturing pc boards the trace width can be calcu-

lated based on a number of variables.

The following equation is reasonably accurate for calculating

the proper trace width for a 50

Ω transmission line.

Power supply decoupling is essential for high frequency op

amps. A 0.01

µF high quality ceramic capacitor at each

supply pin in parallel with a 1

µF tantalum capacitor will

provide excellent decoupling. Chip capacitors produce the

best results due to ease of placement next to the op amp

and they have negligible lead inductance. If leaded capaci-

tors are used, the leads should be kept as short as possible

to minimize lead inductance. The figures that follow illustrate

two different decoupling schemes. Figure 33 improves the

PSRR because the resistor and capacitors create low pass

filters. Note that the supply current will create a voltage drop

across the resistor.

FIGURE 32.

+

50

Ω

50

Ω COAX CABLE

50

Ω

SIGNAL

TRACE

GROUND

PLANE

DIELECTRIC

(PC BOARD)

h

w

t

Z

0

87

E

R

1.41

+

5.98h

0.8 w

t

+

------------------



=

+

V+

1.0

µF

1.0

µF

0.01

µF

0.01

µF

V-