Datasheet

 2010-2011 Microchip Technology Inc. DS22269B-page 25
MCP65R41/6
4.6 PCB Surface Leakage
In applications where the low input bias current is
critical, the Printed Circuit Board (PCB) surface
leakage effects need to be considered. Surface
leakage is caused by humidity, dust or other type of
contamination on the board. Under low humidity
conditions, a typical resistance between nearby traces
is 10
12
. A 5V difference would cause 5 pA of current
to flow. This is greater than the MCP65R41/6 family’s
bias current at +25°C (1 pA, typical).
The easiest way to reduce the surface leakage is to use
a guard ring around the 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 4-9.
FIGURE 4-9: Example of a Guard Ring
Layout for Inverting Circuit.
Use the following steps for an inverting configuration
(Figures 4-6):
1. Connect the guard ring to the non-inverting input
pin (V
IN
+). This biases the guard ring to the
same reference voltage as the comparator
(e.g., V
DD
/2 or ground).
2. Connect the inverting pin (V
IN
-) to the input pad
without touching the guard ring.
Use the following steps for a non-inverting
configuration (Figure 4-4):
1. Connect the non-inverting pin (V
IN
+) to the input
pad without touching the guard ring.
2. Connect the guard ring to the inverting input pin
(V
IN
-).
4.7 Typical Applications
4.7.1 PRECISE COMPARATOR
Some applications require a higher DC precision. A
simple way to address this need is using an amplifier
(such as the MCP6041 – a 600 nA low power and
14 kHz bandwidth op amp) to gain-up the input signal
before it reaches the comparator. Figure 4-10 shows
an example of this approach, which also level shifts to
V
PU
using the Open-Drain option, the MCP65R46.
FIGURE 4-10: Precise Inverting
Comparator.
4.7.2 BISTABLE MULTI-VIBRATOR
A simple bistable multi-vibrator design is shown in
Figure 4-11. V
REF
needs to be between ground and the
maximum comparator internal V
REF
of 2.4V to achieve
oscillation. The output duty cycle changes with V
REF
.
FIGURE 4-11: Bistable Multi-Vibrator.
Guard Ring
V
SS
IN- IN+
V
REF
V
DD
V
DD
R
1
R
2
V
OUT
V
IN
V
REF
V
PU
R
PU
MCP65R46
MCP6041
V
REF
V
DD
R
1
R
2
R
3
V
REF
C
1
V
OUT
MCP65R41