13
FN7384.7
January 16, 2008
Power Supply Bypassing Printed Circuit Board
Layout
As with any high frequency device, a good printed circuit
board layout is necessary for optimum performance. Lead
lengths should be as short as possible. The power supply
pin must be well bypassed to reduce the risk of oscillation.
For normal single supply operation, where the V
S
- pin is
connected to the ground plane, a single 4.7µF tantalum
capacitor in parallel with a 0.1µF ceramic capacitor from V
S
+
to GND will suffice. This same capacitor combination should
be placed at each supply pin to ground if split supplies are to
be used. In this case, the V
S
- pin becomes the negative
supply rail.
Printed Circuit Board Layout
For good AC performance, parasitic capacitance should be
kept to a minimum. Use of wire wound resistors should be
avoided because of their additional series inductance. Use
of sockets should also be avoided if possible. Sockets add
parasitic inductance and capacitance that can result in
compromised performance. Minimizing parasitic capacitance
at the amplifier's inverting input pin is very important. The
feedback resistor should be placed very close to the
inverting input pin. Strip line design techniques are
recommended for the signal traces.
Application Circuits
Sallen Key Low Pass Filter
A common and easy to implement filter taking advantage of
the wide bandwidth, low offset and low power demands of
the EL5150. A derivation of the transfer function is provided
for convenience (see Figure 41).
Sallen Key High Pass Filter
Again, this useful filter benefits from the characteristics of the
EL5150. The transfer function is very similar to the low pass
so only the results are presented (see Figure 42).
FIGURE 41. SALLEN KEY LOW PASS FILTER
K3
1
Q
RC
1
wo
KHolp
CR
CR
CR
CR
CR
CR
)K1(
1
Q
CRCR
1
wo
KHolp
)CRCRCR)K1((jwCRCRw1
1
)jw(H
1s)CRCRCR)K1((sCRCR
K
)s(H
0
sC
1
ViVo
R
VK
Vo
1
R
ViV
V
1sCR
1
KVo
R
R
1K
11
22
12
21
22
11
2211
2221112211
2
2212111
2
2211
1
2
1
1
1
1
22
A
B
−
=
=
=
++−
=
=
=
++−+−
=
+++−+
=
=
−
+
−
+
−
+
=
+=
Equations simplify if we let all
components be equal R = C
+
-
0.1µF
5V
V2
C1
R1
R2
V1
1k
C2
RA
1k
1k
RB
0.1µF
5V
V3
R7
1k
V
OUT
1
3
2
V+
V-
4
U1A
1n
1n
1k
11
EL5150, EL5151, EL5250, EL5251, EL5451
