Microchip Technology MCP6V01DM-VOS データシート
© 2008 Microchip Technology Inc.
DS22058C-page 25
MCP6V01/2/3
After selecting R
ISO
for your circuit, double check the
resulting frequency response peaking and step
response overshoot. Modify R
response overshoot. Modify R
ISO
's value until the
response is reasonable. Bench evaluation and
simulations with the MCP6V01 SPICE macro model
(good for all of the MCP6V01/2/3 op amps) are helpful.
simulations with the MCP6V01 SPICE macro model
(good for all of the MCP6V01/2/3 op amps) are helpful.
4.3.6
STABILIZING OUTPUT LOADS
This family of auto-zeroed op amps has an output
impedance (
impedance (
and
) that has a
double zero when the gain is low. This can cause a
large phase shift in feedback networks that have low
resistance near the part’s bandwidth. This large phase
shift can cause stability problems.
large phase shift in feedback networks that have low
resistance near the part’s bandwidth. This large phase
shift can cause stability problems.
shows one circuit example that has low
resistance near the part’s bandwidth. R
F
and C
F
set a
pole at 0.16 kHz, so the noise gain (G
N
) is 1 V/V at the
circuit’s bandwidth (roughly 1.3 MHz). The load seen
by the op amp’s output at 1.3 MHz is R
by the op amp’s output at 1.3 MHz is R
G
||R
L
(99
Ω).
This is low enough to be a real concern.
FIGURE 4-8:
Output Load Issue.
To solve this problem, increase the resistive load to at
least 3 k
least 3 k
Ω. Methods to accomplish this task include:
• Increase R
G
• Remove C
F
(relocate the filter)
• Add a 3 k
Ω resistor at the op amp’s output that is
not in the signal path; see
FIGURE 4-9:
One Solution To Output
Load Issue.
4.3.7
REDUCING UNDESIRED NOISE
AND SIGNALS
AND SIGNALS
Reduce undesired noise and signals with:
• Low bandwidth signal filters:
• Low bandwidth signal filters:
- Minimizes random analog noise
- Reduces interfering signals
- Reduces interfering signals
• Good PCB layout techniques:
- Minimizes crosstalk
- Minimizes parasitic capacitances and
- Minimizes parasitic capacitances and
inductances that interact with fast switching
edges
edges
• Good power supply design:
- Isolation from other parts
- Filtering of interference on supply line(s)
- Filtering of interference on supply line(s)
4.3.8
SUPPLY BYPASSING AND
FILTERING
FILTERING
With this family of operational amplifiers, the power
supply pin (V
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.
Additional filtering of high frequency power supply
noise (e.g., switched mode power supplies) can be
achieved using resistors. The resistors need to be
small enough to prevent a large drop in V
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.
Additional filtering of high frequency power supply
noise (e.g., switched mode power supplies) can be
achieved using resistors. The resistors need to be
small enough to prevent a large drop in V
DD
for the op
amp, which would cause a reduced output range and
possible load-induced power supply noise. The resis-
tors also need to be large enough to dissipate little
power when V
possible load-induced power supply noise. The resis-
tors also need to be large enough to dissipate little
power when V
DD
is turned on and off quickly. The cir-
gives good rejection out to 1 MHz for
switched mode power supplies. Smaller resistors and
capacitors are a better choice for designs where the
power supply is reasonably quiet.
capacitors are a better choice for designs where the
power supply is reasonably quiet.
FIGURE 4-10:
Additional Supply Filtering.
MCP6V0X
C
F
10.0 k
Ω
R
F
100
Ω
R
G
100
Ω
R
N
V
OUT
V
IN
10.0 k
Ω
R
L
0.1 µF
MCP6V0X
C
F
10.0 k
Ω
R
F
100
Ω
R
G
100
Ω
R
N
V
OUT
V
IN
10.0 k
Ω
R
L
0.1 µF
3.01 k
Ω
R
X
MCP6V0X
V
S_ANA
143
Ω
143
Ω
100 µF
100 µF
0.1 µF
1/4W
1/10W
to other analog parts