Datasheet

 2010-2011 Microchip Technology Inc. DS22243D-page 19
MCP6H01/2/4
4.6 PCB Surface Leakage
In applications where low input bias current is critical,
PCB surface leakage effects need to be considered.
Surface leakage is caused by humidity, dust or other
contamination on the board. Under low-humidity condi-
tions, a typical resistance between nearby traces is
10
12
. A 15V difference would cause 15 pA of current
to flow; which is greater than the MCP6H01/2/4 family’s
bias current at +25°C (10 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 Trans-impedance 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 DIFFERENCE AMPLIFIER
The MCP6H01/2/4 op amps can be used in current
sensing applications. Figure 4-8 shows a resistor
(R
SEN
) that converts the sensor current (I
SEN
) to
voltage, as well as a difference amplifier that amplifies
the voltage across the resistor while rejecting common
mode noise. R
1
and R
2
must be well matched to obtain
an acceptable Common Mode Rejection Ratio
(CMRR). Moreover, R
SEN
should be much smaller than
R
1
and R
2
in order to minimize the resistive loading of
the source.
To ensure proper operation, the op amp common mode
input voltage must be kept within the allowed range.
The reference voltage (V
REF
) is supplied by a
low-impedance source. In single-supply applications,
V
REF
is typically V
DD
/2.
.
FIGURE 4-8: High Side Current Sensing
Using Difference Amplifier.
Guard Ring V
IN
–V
IN
+ V
SS
R
1
V
OUT
R
2
R
1
R
SEN
I
SEN
R
SEN
<< R
1
, R
2
V
OUT
V
1
V
2
–
R
2
R
1
------


V
REF
+=
V
REF
R
2
V
DD
MCP6H01