Datasheet

© 2010 Microchip Technology Inc. DS22182B-page 17
MCP6051/2/4
4.6 PCB Surface Leakage
In applications where low input bias current is critical,
Printed Circuit Board (PCB) surface leakage effects
need to be considered. Surface leakage is caused by
humidity, dust or other contamination on the board.
Under low humidity conditions, a typical resistance
between nearby traces is 10
12
Ω. A 5V difference would
cause 5 pA of current to flow; which is greater than the
MCP6051/2/4 family’s bias current at +25°C (±1.0 pA,
typical).
The easiest way to reduce surface leakage is to use a
guard ring around sensitive pins (or traces). The guard
ring is biased at the same voltage as the sensitive pin.
An example of this type of layout is shown in
Figure 4-7.
FIGURE 4-7: Example Guard Ring Layout
for Inverting Gain.
1. Non-inverting Gain and Unity-Gain Buffer:
a) Connect the non-inverting pin (V
IN
+) to the
input with a wire that does not touch the
PCB surface.
b) Connect the guard ring to the inverting input
pin (V
IN
–). This biases the guard ring to the
common mode input voltage.
2. Inverting Gain and Transimpedance Gain
Amplifiers (convert current to voltage, such as
photo detectors):
a) Connect the guard ring to the non-inverting
input pin (V
IN
+). This biases the guard ring
to the same reference voltage as the op
amp (e.g., V
DD
/2 or ground).
b) Connect the inverting pin (V
IN
–) to the input
with a wire that does not touch the PCB
surface.
4.7 Application Circuits
4.7.1 GYRATOR
The MCP6051/2/4 op amps can be used in gyrator
applications. The gyrator is an electric circuit which can
make a capacitive circuit behave inductively.
Figure 4-8 shows an example of a gyrator simulating
inductance, with an approximately equivalent circuit
below. The two Z
IN
have similar values in typical
applications. The primary application for a gyrator is to
reduce the size and cost of a system by removing the
need for bulky, heavy and expensive inductors. For
example, RLC bandpass filter characteristics can be
realized with capacitors, resistors and operational
amplifiers without using inductors. Moreover, gyrators
will typically have higher accuracy than real inductors,
due to the lower cost of precision capacitors than
inductors.
.
FIGURE 4-8: Gyrator.
Guard Ring V
IN
–V
IN
+ V
SS
R
L
V
OUT
Gyrator
Z
IN
R
C
Z
IN
R
L
j ω L+=
LR
L
RC=
R
L
L
Z
IN
Equivalent Circuit
MCP6051