MCP6272 Microchip Technology Inc., MCP6272 Datasheet - Page 14

• Manufacturer Part Number: MCP6272
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Am
• Manufacturer: Microchip Technology Inc.

MCP6271/1R/2/3/4/5
4.6
An unused op amp in a quad package (MCP6274)
should be configured as shown in
circuits prevent the output from toggling and causing
crosstalk. In Circuit A, R
within its output voltage range (V
buffers this voltage, which can be used elsewhere in
the circuit. Circuit B uses the minimum number of
components and operates as a comparator.
FIGURE 4-6:
4.7
With this family of operational amplifiers, the power
supply pin (V
bypass capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm
for good, high frequency performance. It also needs a
bulk capacitor (i.e., 1 µF or larger) within 100 mm to
provide large, slow currents. This bulk capacitor can be
shared with nearby analog parts.
4.8
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. This is greater than the
MCP6271/1R/2/3/4/5 family’s bias current at 25°C
(1 pA, typ.).
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 illustrated in
Figure
DS21810E-page 14
¼ MCP6274 (A)
V
DD
4-7.
R
R
1
2
Unused Amplifiers
Supply Bypass
PCB Surface Leakage
DD
V
for single supply) should have a local
DD
Unused Op Amps.
1
and R
12
Ω. A 5V difference would
OH
2
¼ MCP6274 (B)
, V
produce a voltage
Figure
OL
V
). The op amp
DD
4-6. These
FIGURE 4-7:
for Inverting Gain.
1.
2.
For
Amplifiers (convert current to voltage, such as
photo detectors):
a)
b)
Non-inverting Gain and Unity Gain Buffer:
a)
b)
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.
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.
Inverting
IN
–). This biases the guard ring to the
V
IN
IN
DD
Guard Ring
Gain
+). This biases the guard ring
Example Guard Ring Layout
–
© 2006 Microchip Technology Inc.
/2 or ground).
and
V
IN
+
Transimpedance
IN
–) to the input
IN
V
SS
+) to the