MCP6071 Microchip Technology Inc., MCP6071 Datasheet - Page 17

• Manufacturer Part Number: MCP6071
• Description: 110 ?a, High Precision Op Amps
• Manufacturer: Microchip Technology Inc.

4.6
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
cause 5 pA of current to flow; which is greater than the
MCP6071/2/4 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
FIGURE 4-6:
for Inverting Gain.
1.
2.
© 2009 Microchip Technology Inc.
Non-inverting Gain and Unity-Gain Buffer:
a.
b.
Inverting Gain and Transimpedance Gain Ampli-
fiers (convert current to voltage, such as photo
detectors):
a.
b.
Guard Ring
4-6.
PCB Surface Leakage
Connect the non-inverting pin (V
input with a wire that does not touch the
PCB surface.
Connect the guard ring to the inverting input
pin (V
common mode input voltage.
Connect the guard ring to the non-inverting
input pin (V
to the same reference voltage as the op
amp (e.g., V
Connect the inverting pin (V
with a wire that does not touch the PCB
surface.
IN
–). This biases the guard ring to the
IN
DD
+). This biases the guard ring
Example Guard Ring Layout
/2 or ground).
V
IN
12
– V
Ω. A 5V difference would
IN
+
IN
–) to the input
V
SS
IN
+) to the
MCP6071/2/4
DS22142A-page 17