Datasheet

© 2009-2011 Microchip Technology Inc. DS22229D-page 19
MCP6401/1R/1U/2/4/6/7/9
4.6 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 of current to flow; which is greater than the
MCP6401/1R/1U/2/4/6/7/9 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 4-7.
FIGURE 4-7: 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.
4.7 Application Circuits
4.7.1 PRECISION HALF-WAVE
RECTIFIER
The precision half-wave rectifier, which is also known
as a super diode, is a configuration obtained with an
operational amplifier in order to have a circuit behave
like an ideal diode and rectifier. It effectively cancels the
forward voltage drop of the diode so that very low level
signals can still be rectified with minimal error. This can
be useful for high-precision signal processing. The
MCP6401/1R/1U/2/4/6/7/9 op amps have high input
impedance, low input bias current and rail-to-rail
input/output, which makes this device suitable for
precision rectifier applications.
Figure 4-8 shows a precision half-wave rectifier and its
transfer characteristic. The rectifier’s input impedance
is determined by the input resistor R
1
. To avoid loading
effect, it must be driven from a low-impedance source.
When V
IN
is greater than zero, D
1
is OFF, D
2
is ON, and
V
OUT
is zero. When V
IN
is less than zero, D
1
is ON, D
2
is OFF, and V
OUT
is the V
IN
with an amplification of
-R
2
/R
1
.
The rectifier circuit shown in Figure 4-8 has the benefit
that the op amp never goes in saturation, so the only
thing affecting its frequency response is the
amplification and the gain bandwidth product.
.
FIGURE 4-8: Precision Half-Wave
Rectifier.
Guard Ring V
IN
–V
IN
+ V
SS
V
OUT
R
2
D
1
D
2
R
1
V
IN
V
OUT
V
IN
-R
2
/R
1
Transfer Characteristic
Precision Half-Wave Rectifier
MCP6401