MCP6231/2
10k
Recommended R
ISO
( )
1.E+04
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
G
N
= 1 V/V
G
N
= 2 V/V
G
N
≥
4 V/V
1k
1.E+03
V
IN
–
V
IN
+
V
SS
100
10p
1.E+02
1.E+01
1.E+02
1.E+03
1.E+04
100p
1n
10n
Normalized Load Capacitance; C
L
/G
N
(F)
FIGURE 3-4:
Recommended R
ISO
Values
for Capactive Loads.
After selecting R
ISO
for your circuit, double-check the
resulting frequency response peaking and step
response overshoot. Evaluation on the bench and
simulations with the MCP6231/2 SPICE macro model
are very helpful. Modify R
ISO
’s value until the response
is reasonable.
Guard Ring
FIGURE 3-5:
for Inverting Gain.
1.
Example Guard Ring Layout
3.4
Supply Bypass
With this op amp, the power supply pin (V
DD
for
single-supply) should have a local 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
other parts.
2.
3.5
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, if current-to-flow. This is greater than the
MCP6231/2 family’s bias current at 25°C (1 pA, typ).
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.
Inverting 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.
DS21881B-page 8
2004 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
