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
without R
2
and with R
I
shorted. R
1
should then be 1M
to
10M
, 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
lines are common in
communications and 75
lines in video systems. Impedance
matching is important to minimize reflected energy therefore
minimizing
transmitted
signal
accomplished by using a series matching resistor (50
or
75
), matched transmission line (50
or 75
), 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
±
20% resistor connected to
the op amp’s output with the other end of the resistor pinned
out. This 50
resistor can be used as the series resistor
instead of an external resistor.
distortion.
This
is
50
+
R
I
50K
R
1
100K
+5V
V
OUT
100
R
2
-5V
V
IN
R
F
Adjustment Range
V
R2
R1
-----
±
+
50K
+V
-V
R
1
100K
100
R
2
V
OUT
V
IN
R
F
R
I
Adjustment Range
V
R2
R1
----
±
Gain
1
RF
+
RI
R2
---------------
+
FIGURE 30.
PC board traces can be made to look like a 50
or 75
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.
SIGNAL
TRACE
w
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
transmission line.
+
Power supply decoupling is essential for high frequency op
amps. A 0.01
μ
F high quality ceramic capacitor at each
supply pin in parallel with a 1
μ
F 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.
V+
FIGURE 32.
+
V
OUT
V
IN
R
F
50
50
COAX CABLE
50
GROUND
PLANE
DIELECTRIC
(PC BOARD)
h
t
E
R
Z0
ER
1.41
-------------------------
In
0.8 w
t
+
-----------------
=
+
1.0
μ
F
1.0
μ
F
0.01
μ
F
0.01
μ
F
V-