Datasheet

© 2009 Microchip Technology Inc. DS21881E-page 13
MCP6231/1R/1U/2/4
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
MCP6231/1R/1U/2/4 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 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 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 MATCHING THE IMPEDANCE AT
THE INPUTS
To minimize the effect of input bias current in an ampli-
fier circuit (this is important for very high source-
impedance applications, such as pH meters and
transimpedance amplifiers), the impedances at the
inverting and non-inverting inputs need to be
matched. This is done by choosing the circuit resistor
values so that the total resistance at each input is the
same. Figure 4-8 shows a summing amplifier circuit.
FIGURE 4-8: Summing Amplifier Circuit.
To match the inputs, set all voltage sources to ground
and calculate the total resistance at the input nodes. In
this summing amplifier circuit, the resistance at the
inverting input is calculated by setting V
IN1
, V
IN2
and
V
OUT
to ground. In this case, R
G1
, R
G2
and R
F
are in
parallel. The total resistance at the inverting input is:
EQUATION 4-1:
At the non-inverting input, V
DD
is the only voltage
source. When V
DD
is set to ground, both R
x
and R
y
are
in parallel. The total resistance at the non-inverting
input is:
EQUATION 4-2:
Guard Ring
V
SS
V
IN
–V
IN
+
MCP623X
V
OUT
V
IN2
–
+
V
IN1
R
G2
R
G1
R
F
R
Z
V
DD
R
X
R
Y
R
VIN
–
1
1
R
G1
---------
1
R
G2
---------
1
R
F
------++
⎝⎠
⎛⎞
----------------------------------------------=
Where:
R
VIN
–
= total resistance at the inverting
input
R
VIN
+
1
1
R
X
------
1
R
Y
------+
⎝⎠
⎛⎞
--------------------------R
Z
+=
Where:
R
VIN
+
= total resistance at the inverting
input