MCP6234 Microchip Technology, MCP6234 Datasheet - Page 10

• Manufacturer Part Number: MCP6234
• Description: 300 kHz Rail-to-Rail Op Amp
• Manufacturer: Microchip Technology

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MCP6231/2/4
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 4-7 shows a summing amplifier circuit.
FIGURE 4-7:
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:
DS21881C-page 10
OUT
Where:
Where:
V
V
to ground. In this case, R
R
R
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
VIN +
input
G1
G2
–
is set to ground, both R
R
=
Z
=
-------------------------------------------- -
⎛
⎝
Summing Amplifier Circuit.
---------
R
------------------------ -
⎛
⎝
1
G1
------
R
MCP623X
1
–
+
X
+
1
+
R
---------
R
DD
1
----- -
R
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
To minimize output offset voltage and increase circuit
accuracy, the resistor values need to meet the
conditions:
4.7.2
In analog circuit design, the PCB parasitic capacitance
can compromise the circuit behavior; Figure 4-8 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,
typ.), and large R
the circuit will include a zero. This parasitic zero
introduces gain-peaking and can cause circuit
instability.
FIGURE 4-8:
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
F
and R
R
VIN +
Effect of Parasitic
© 2005 Microchip Technology Inc.
MCP623X
+
–
G
PARA
, the frequency response of
=
R
C
F
F
R
IN
:C
C
–
F
F
is equal to the ratio
=
F
). C
C
PARA
F
V
needs to be
OUT
R
------ -
R
G
F
PARA
F
in
,