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MCP617 Datasheet PDF : 38 Pages
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MCP616/7/8/9
4.8 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 1012Ω. A 5V difference would
cause 5 pA of current to flow, which is greater than the
MCP616/7/8/9 family’s bias current at 25°C
(1 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 is shown below in Figure 4-8.
Guard Ring VIN– VIN+ VSS
FIGURE 4-8:
Example Guard Ring Layout
for Inverting Gain.
1. Non-inverting Gain and Unity Gain Buffer:
a) Connect the non-inverting pin (VIN+) to the
input with a wire that does not touch the
PCB surface.
b) Connect the guard ring to the inverting input
pin (VIN–). This biases the guard ring to the
common mode input voltage.
2. Inverting Gain and Transimpedance gain (con-
vert current to voltage, such as photo detectors)
amplifiers:
a) Connect the guard ring to the non-inverting
input pin (VIN+). This biases the guard ring
to the same reference voltage as the op
amp (e.g., VDD/2 or ground).
b) Connect the inverting pin (VIN–) to the input
with a wire that does not touch the PCB
surface.
4.9 Application Circuits
4.9.1 HIGH GAIN PRE-AMPLIFIER
The MCP616/7/8/9 op amps are well suited to
amplifying small signals produced by low-impedance
sources/sensors. The low offset voltage, low offset
current and low noise fit well in this role. Figure 4-9
shows a typical pre-amplifier connected to a low-
impedance source (VS and RS).
RS
VS
10 kΩ
VDD/2
RG
11.0 kΩ
MCP616
RF
100 kΩ
VOUT
FIGURE 4-9:
High Gain Pre-amplifier.
For the best noise and offset performance, the source
resistance RS needs to be less than 15 kΩ. The DC
resistances at the inputs are equal to minimize the
offset voltage caused by the input bias currents
(Section 4.2 “DC Offsets”). In this circuit, the DC gain
is 10 V/V, which will give a typical bandwidth of 19 kHz.
4.9.2
TWO OP AMP INSTRUMENTATION
AMPLIFIER
The two-op amp instrumentation amplifier shown in
Figure 4-10 serves the function of taking the difference
of two input voltages, level-shifting it and gaining it to
the output. This configuration is best suited for higher
gains (i.e., gain > 3 V/V). The reference voltage (VREF)
is typically at mid-supply (VDD/2) in a single-supply
environment.
VO U T
=
(V1
–
⎛
V2
)
⎜1
⎝
+
R----1--
R2
+
2----R----1-
RG
⎞
⎟
⎠
+
VREF
RG
R1
R2
R2
R1
VREF
VOUT
V2
½
½
MCP617
MCP617
V1
FIGURE 4-10:
Two-Op Amp
Instrumentation Amplifier.
The key specifications that make the MCP616/7/8/9
family appropriate for this application circuit are low
input bias current, low offset voltage and high common-
mode rejection.
DS21613C-page 18
© 2008 Microchip Technology Inc.
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