MCP6071 Microchip Technology Inc., MCP6071 Datasheet - Page 16

• Manufacturer Part Number: MCP6071
• Description: 110 ?a, High Precision Op Amps
• Manufacturer: Microchip Technology Inc.

MCP6071/2/4
4.3
Driving large capacitive loads can cause stability
problems for voltage feedback op amps. As the load
capacitance increases, the feedback loop’s phase
margin decreases and the closed-loop bandwidth is
reduced. This produces gain peaking in the frequency
response, with overshoot and ringing in the step
response. While a unity-gain buffer (G = +1) is the most
sensitive to capacitive loads, all gains show the same
general behavior.
When driving large capacitive loads with these op
amps (e.g., > 100 pF when G = +1), a small series
resistor at the output (R
feedback loop’s phase margin (stability) by making the
output load resistive at higher frequencies. The
bandwidth will be generally lower than the bandwidth
with no capacitance load.
FIGURE 4-3:
Stabilizes Large Capacitive Loads.
Figure 4-4
different capacitive loads and gains. The x-axis is the
normalized load capacitance (C
circuit's noise gain. For non-inverting gains, G
Signal Gain are equal. For inverting gains, G
1+|Signal Gain| (e.g., -1 V/V gives G
FIGURE 4-4:
for Capacitive Loads.
DS22142A-page 16
V
IN
1000
100
10
1.E-11
1
10p
Capacitive Loads
gives recommended R
–
MCP607X
+
Normalized Load Capacitance; C
1.E-10
100p
Output Resistor, R
Recommended R
ISO
1.E-09
1n
G
1 V/V
2 V/V
≥ 5 V/V
N
in
:
Figure
R
1.E-08
L
10n
ISO
/G
C
N
), where G
N
L
4-3) improves the
ISO
= +2 V/V).
V
R
1.E-07
DD
L
0.1 µ
= 10 k Ω
L
= 6.0 V
/G
ISO
values for
ISO
N
(F)
N
Values
V
1.E-06
and the
N
OUT
1µ
is the
N
is
After selecting R
resulting frequency response peaking and step
response overshoot. Modify R
response is reasonable. Bench evaluation and simula-
tions with the MCP6071/2/4 SPICE macro model are
very helpful.
4.4
With this family of operational amplifiers, the power
supply pin (V
bypass capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm
for good high frequency performance. It can use 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 analog parts.
4.5
An unused op amp in a quad package (MCP6074)
should be configured as shown in
circuits prevent the output from toggling and causing
crosstalk. Circuits A sets the op amp at its minimum
noise gain. The resistor divider produces any desired
reference voltage within the output voltage range of the
op amp; the op amp buffers that reference voltage.
Circuit B uses the minimum number of components
and operates as a comparator, but it may draw more
current.
FIGURE 4-5:
V
DD
¼ MCP6074 (A)
V REF
R
R
1
2
Supply Bypass
Unused Op Amps
=
DD
V DD
V
DD
for single-supply) should have a local
ISO
×
for your circuit, double-check the
--------------------
R 1
Unused Op Amps.
R 2
© 2009 Microchip Technology Inc.
+
V
R 2
REF
ISO
’s value until the
Figure
¼ MCP6074 (B)
V
4-5. These
DD