DATA SHEET
KH104
PC Board Layout Considerations
Proper layout of printed circuit boards is important to
achieve optimum performance of a circuit operating in
the 1GHz frequency range. Use of microstripline is
recommended for all signal-carrying paths and low
resistance, low inductance signal return and bypass
paths should be used. To keep the impedance of
these paths low, use as much ground plane as possible.
Ground plane also serves to increase the flow of heat out
of the package.
The KH104 has three types of connections: signal paths
(input and output), DC inputs (supplies and offset adjust),
and grounds. 50Ω microstrip is recommended for
connection to the input (pin 4) and output (pin 11).
Microstrip on a doublesided PC board consists of a
ground plane on one side of the board and a constant-
width signal-carrying trace on the other side of the board.
For 1/16” G10 or FR-4 PC board material, a 0.1” wide
trace will have a 50Ω characteristic impedance. The
ground plane beneath the signal trace must extend at
least one trace width on either side of the trace. Also, all
traces (including ground) should be kept at least one
trace width from the signal carrying traces.
To keep power supply noise and oscillations from
appearing at the amplifier output, all supply pins should
be capacitively bypassed to ground. The power
supply pins (1 and 2) are the inputs to a pair of voltage
regulators whose outputs are at pins 13 and 14. It is
recommended that 0.01µF or larger ceramic capacitors
be connected from pins 1, 2, 13 and 14 to ground, within
0.2” of the pins. A 1µF or larger solid tantalum capacitor
to ground is required within 3” of pins 1 and 2, and
for good low frequency performance, solid tantalum
capacitors of at least 15µF should be connected from
pins 13 and 14 to ground within 3” of the pins. Use
0.025” or wider traces for the supply lines. The offset
adjust pin (12) also requires bypassing; a 0.01µF or
larger ceramic capacitor to ground within 0.2” of the pin
is recommended.
Grounding is the final layout consideration. Pins 3 and 5-
10 should all be connected to a ground plane which
should cover as much of one side of the board around
the amplifier as possible.
Reducing DC Offset
DC offset of the KH104 may be adjusted by applying a
DC voltage to the amplifier’s offset adjust pin (12). The
simplest method is shown in Figure 1. Using this method
of offset adjust it is possible to vary the output offset by
approximately ±400mV. This simple adjustment has no
effect on the offset drift characteristics of the KH104.
+15V
39
0.01
+15V
2.2
10K
Offset
Adjust
-15V
V
in
0.01
0.01
12
4
3,5-10
14
1
KH104
2
13
11
V
o
0.01
2.2
Capacitance if
µF
39
0.01
-15V
Figure 1: Basic Circuit
If lower offset and offset drift are required, a low frequency
op amp may be used in conjunction with the KH104 in a
composite configuration. The suggested circuit appears
in Figure 2. Its method of operation is to compare an
attenuated version of the output signal to the input signal
and apply a correcting voltage at the offset adjust pin. A
compensation capacitor C
s
reduces the bandwidth of the
op amp correction circuit to limit the op amp’s effect on
the KH104 to frequencies below f
45
, the frequency at
which the op amp has 45dB of open loop gain. Using an
LM108, f
45
is about 7Hz with C
s
= 0.1µF. Thus the op
amp can correct DC and low frequency errors below f
45
,
without affecting KH104 performance above f
45
. Also
note that the noise performance of the op amp will dom-
inate below f
45
.
12
4
11
+15V
0.01
2k
7
2
6
V
in
KH104
V
o
R
L
50Ω
49.9k
0.01
R
c
9.76k
R
a
11.8k
R
b
1k
LM108
8
3
4
C
s
0.01
0.01
Capacitance in
µF
R
c
= (R
a
+ R
b
) || 49.9k
-15V
Figure 2: Composite Amplifier
K
With an LM108 op amp in this composite configuration,
input offset is typically 2mV and drift is 15mV/°C. At
frequencies well below f
45
, the composite gain is equal
to (1 + 49.9k/(R
a
+ R
b
)) and the output impedance is
4
REV. 1A January 2004
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
