Datasheet

© 2009 Microchip Technology Inc. DS21668D-page 17
MCP6141/2/3/4
Figure 4-5 shows three example circuits that are
unstable when used with the MCP6141/2/3/4 family.
The unity gain buffer and low gain amplifier
(non-inverting or inverting) are at gains that are too low
for stability (see Equation 4-2).The Miller integrator’s
capacitor makes it reach unity gain at high frequencies,
causing instability.
FIGURE 4-5: Examples of Unstable
Circuits for the MCP6141/2/3/4 Family.
4.4.2 CAPACITIVE LOADS
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. A unity gain buffer (G = +1) is the most
sensitive to capacitive loads, though all gains show the
same general behavior.
When driving large capacitive loads with these op
amps (e.g., > 60 pF when G = +10), a small series
resistor at the output (R
ISO
in Figure 4-6) improves the
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 capacitive load.
FIGURE 4-6: Output Resistor, R
ISO
stabilizes large capacitive loads.
Figure 4-7 gives recommended R
ISO
values for
different capacitive loads and gains. The x-axis is the
normalized load capacitance (C
L
/G
N
), where G
N
is the
circuit’s noise gain. For non-inverting gains, G
N
and the
Signal Gain are equal. For inverting gains, G
N
is
1 + |Signal Gain| (e.g., -9 V/V gives G
N
= +10 V/V).
FIGURE 4-7: Recommended R
ISO
Values
for Capacitive Loads.
After selecting R
ISO
for your circuit, double check the
resulting frequency response peaking and step
response overshoot. Modify R
ISO
’s value until the
response is reasonable. Bench evaluation and
simulations with the MCP6141/2/3/4 SPICE macro
model are helpful.
4.5 MCP6143 Chip Select
The MCP6143 is a single op amp with Chip Select
(CS
). When CS is pulled high, the supply current drops
to 50 nA (typical) and flows through the CS pin to V
SS
.
When this happens, the amplifier output is put into a
high impedance state. By pulling CS
low, the amplifier
is enabled. If the CS
pin is left floating, the amplifier will
not operate properly. Figure 1-1 shows the output
voltage and supply current response to a CS
pulse.
Note: The three circuits shown in Figure 4-5 are
not to be used with the MCP6141/2/3/4 op
amps. They are included for illustrative
purposes only.
Low Gain Amplifier
MCP614X
V
OUT
R
G
V
1
R
F
R
N
1
R
F
R
G
-------
10<+
V
2
MCP614X
V
OUT
V
IN
MCP614X
V
OUT
CR
V
IN
Unity Gain Buffer
Miller Integrator
V
B
MCP614X
R
ISO
V
OUT
C
L
R
F
R
G
V
A
1,000
10,000
100,000
1.E+00 1.E+01 1.E+02 1.E+03
Normalized Load Capacitance; C
L
/G
N
(F)
Recommended R
ISO
(Ω)
1p
1k
100k
10p
G
N
= +10
G
N
= +20
G
N
≥ +50
10k
1n100p