Datasheet

© 2010-2012 Microchip Technology Inc. DS22257C-page 15
MCP6441/2/4
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 4-6.
FIGURE 4-6: 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.
4.6 Application Circuits
4.6.1 BATTERY CURRENT SENSING
The MCP6441/2/4 op amp’s Common Mode Input
Range, which goes 0.3V beyond both supply rails,
supports their use in high-side and low-side battery
current sensing applications. The low quiescent current
(450 nA, typical) helps prolong battery life, and the
rail-to-rail output supports detection of low currents.
Figure 4-7 shows a high side battery current sensor
circuit. The 10Ω resistor is sized to minimize power
losses. The battery current (I
DD
) through the 10Ω
resistor causes its top terminal to be more negative
than the bottom terminal. This keeps the Common
Mode input voltage of the op amp below V
DD
, which is
within its allowed range. The output of the op amp will
also be below V
DD
, within its Maximum Output Voltage
Swing specification.
FIGURE 4-7: Battery Current Sensing.
Guard Ring V
IN
–V
IN
+ V
SS
V
DD
I
DD
100 kΩ
1MΩ
1.4V
V
OUT
10Ω
to
6.0V
I
DD
V
DD
V
OUT
–
10 V/V()10
Ω () ⋅
----------------------------------------- -=
To load
MCP6441