MCP6232-E/SNG Microchip Technology, MCP6232-E/SNG Datasheet - Page 12

• Manufacturer Part Number: MCP6232-E/SNG
• Description: 1.8V, 200KHz DUAL LOW-COST CMOS OPERATIONAL AMPLIFIER, -40C to +125C, 8-SOIC 150mil, TUBE
• Manufacturer: Microchip Technology

MCP6231/1R/1U/2/4
4.6
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
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
FIGURE 4-7:
for Inverting Gain.
1.
2.
DS21881D-page 12
Non-inverting Gain and Unity-Gain Buffer:
a.
b.
Inverting Gain and Transimpedance Amplifiers
(convert current to voltage, such as photo
detectors):
a.
b.
4-7.
PCB Surface Leakage
Connect the non-inverting pin (V
input with a wire that does not touch the
PCB surface.
Connect the guard ring to the inverting input
pin (V
common mode input voltage.
Connect the guard ring to the non-inverting
input pin (V
to the same reference voltage as the op
amp (e.g., V
Connect the inverting pin (V
with a wire that does not touch the PCB
surface.
IN
–). This biases the guard ring to the
V
IN
IN
Guard Ring
DD
+). This biases the guard ring
Example Guard Ring Layout
–
/2 or ground).
12
Ω. A 5V difference would
V
IN
+
IN
–) to the input
IN
V
SS
+) to the
4.7
4.7.1
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:
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
V
parallel. The total resistance at the inverting input is:
At the non-inverting input, V
source. When V
in parallel. The total resistance at the non-inverting
input is:
To minimize output offset voltage and increase circuit
accuracy, the resistor values need to meet the
conditions:
OUT
Where:
Where:
V
V
to ground. In this case, R
R
R
Figure 4-8
IN2
IN1
VIN –
VIN +
Application Circuits
R
R
X
Y
MATCHING THE IMPEDANCE AT
THE INPUTS
V
= total resistance at the inverting input
= total resistance at the inverting
DD
R
R
R
R
IN
DD
input
G1
VIN +
G2
shows a summing amplifier circuit.
–
is set to ground, both R
R
=
R
Z
VIN +
=
Summing Amplifier Circuit.
-------------------------------------------- -
⎛
⎝
---------
R
© 2008 Microchip Technology Inc.
------------------------ -
⎛
⎝
1
G1
----- -
R
1
MCP623X
–
+
X
=
+
1
+
R
R
---------
R
1
DD
----- -
R
IN
F
1
1
G2
G1
Y
⎞
⎠
–
+
, R
is the only voltage
+
----- -
R
R
1
G2
F
Z
⎞
⎠
and R
IN1
x
V
and R
, V
OUT
F
IN2
are in
y
and
are