MCP6242 Microchip Technology, MCP6242 Datasheet - Page 9

• Manufacturer Part Number: MCP6242
• Description: (MCP6241 / MCP6242) Rail-to-Rail Op Amp
• Manufacturer: Microchip Technology

4.0
4.1
To minimize the effect of offset voltage in an amplifier
circuit, the impedance at both inverting and non-
inverting inputs needs to be matched. This is done by
choosing the circuit resistor values so that the total
resistance at each input is the same. Figure 4-1 shows
a summing amplifier circuit.
FIGURE 4-1:
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 offset voltage and increase circuit
accuracy, the resistor values need to meet the
condition:
 2004 Microchip Technology Inc.
OUT
Where:
Where:
to ground. In this case, R
V
V
R
R
IN2
IN1
VIN –
VIN +
APPLICATION CIRCUITS
Matching the Impedance at the
Inputs
R
R
Y
X
V
= total resistance at the inverting input
= total resistance at the inverting
R
DD
R
R
VIN -
R
DD
VIN +
input
G1
G2
is set to ground, both R
R
R
=
Z
VIN +
=
Summing Amplifier Circuit.
-------------------------------------------- -


---------
R
------------------------ -


1
G1
------
R
MCP624X
1
–
+
=
X
+
+
1
R
R
---------
R
DD
----- -
R
VIN -
1
F
1
1
G2
G1
Y


, R
is the only voltage
+
+
----- -
R
R
1
G2
F
Z


and R
IN1
X
and R
V
, V
OUT
F
IN2
are in
Y
and
are
4.2
In analog circuit design, the PCB parasitic capacitance
can compromise the circuit behavior; Figure 4-2 shows
a typical scenario. If the input of an amplifier sees
parasitic capacitance of several picofarad (C
which includes the common mode capacitance of 6 pF,
typical) and large RF and RG, the frequency response
of the circuit will include a zero. This parasitic zero
introduces gain peaking and can cause circuit
instability.
FIGURE 4-2:
Capacitance at the Input.
One solution is to use smaller resistor values to push
the zero to a higher frequency. Another solution is to
compensate by introducing a pole at the point at which
the zero occurs. This can be done by adding C
parallel with the feedback resistor (R
selected so that the ratio C
of R
V
V
F
AC
DC
:R
G
.
Compensating for the Parasitic
Capacitance
C
R
PARA
G
Effect of Parasitic
MCP624X
+
–
MCP6241/2
PARA
R
C
F
F
:C
C
F
F
is equal to the ratio
=
F
). C
DS21882B-page 9
C
PARA
F
V
needs to be
OUT
R
------ -
R
G
F
PARA
F
in
,