Datasheet
© 2012 Microchip Technology Inc. DS25124A-page 23
MCP6V11/1U
4.3.8 GAIN PEAKING
Figure 4-10 shows an op amp circuit that represents
non-inverting amplifiers (V
M
is a DC voltage and V
P
is
the input) or inverting amplifiers (V
P
is a DC voltage
and V
M
is the input). The capacitances C
N
and C
G
rep-
resent the total capacitance at the input pins; they
include the op amp’s common mode input capacitance
(C
CM
), board parasitic capacitance and any capacitor
placed in parallel. The capacitance C
FP
represents the
parasitic capacitance coupling the output and non-
inverting input pins.
FIGURE 4-10: Amplifier with Parasitic
Capacitance.
C
G
acts in parallel with R
G
(except for a gain of +1 V/V),
which causes an increase in gain at high frequencies.
C
G
also reduces the phase margin of the feedback
loop, which becomes less stable. This effect can be
reduced by either reducing C
G
or R
F
||R
G
.
C
N
and R
N
form a low-pass filter that affects the signal
at V
P
. This filter has a single real pole at 1/(2πR
N
C
N
).
The largest value of R
F
that should be used depends
on noise gain (see G
N
in Section 4.3.6, Capacitive
Loads), C
G
and the open-loop gain’s phase shift. An
approximate limit for R
F
is:
EQUATION 4-2:
Some applications may modify these values to reduce
either output loading or gain peaking (step response
overshoot).
At high gains, R
N
needs to be small, in order to prevent
positive feedback and oscillations. Large C
N
values
can also help.
4.3.9 REDUCING UNDESIRED NOISE
AND SIGNALS
Reduce undesired noise and signals with:
• Low bandwidth signal filters:
- Minimizes random analog noise
- Reduces interfering signals
• Good PCB layout techniques:
- Minimizes crosstalk
- Minimizes parasitic capacitances and
inductances that interact with fast switching
edges
• Good power supply design:
- Isolation from other parts
- Filtering of interference on supply line(s)
4.3.10 SUPPLY BYPASSING AND
FILTERING
With this family of operational amplifiers, the power
supply pin (V
DD
for single supply) should have a local
bypass capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm
of the pin for good high-frequency performance.
These parts also need a bulk capacitor (i.e., 1 µF or
larger) within 100 mm to provide large, slow currents.
This bulk capacitor can be shared with other low noise,
analog parts.
In some cases, high-frequency power supply noise
(e.g., switched mode power supplies) may cause
undue intermodulation distortion, with a DC offset shift;
this noise needs to be filtered. Adding a resistor into the
supply connection can be helpful.
4.3.11 PCB DESIGN FOR DC PRECISION
In order to achieve DC precision on the order of ±1 µV,
many physical errors need to be minimized. The design
of the Printed Circuit Board (PCB), the wiring, and the
thermal environment have a strong impact on the
precision achieved. A poor PCB design can easily be
more than 100 times worse than the MCP6V11/1U op
amps’ minimum and maximum specifications.
4.3.11.1 PCB Layout
Any time two dissimilar metals are joined together, a
temperature dependent voltage appears across the
junction (the Seebeck or thermojunction effect). This
effect is used in thermocouples to measure
temperature. The following are examples of
thermojunctions on a PCB:
• Components (resistors, op amps, …) soldered to
a copper pad
• Wires mechanically attached to the PCB
• Jumpers
• Solder joints
•PCB vias
R
G
R
F
V
OUT
U
1
MCP6V1X
C
G
R
N
C
N
V
M
V
P
C
FP
R
F
40 k
Ω
()
12 pF
C
G
--------------
×
G
N
2
×≤