Datasheet
MCP6401/1R/1U/2/4/6/7/9
DS22229D-page 18 © 2009-2011 Microchip Technology Inc.
4.2 Rail-to-Rail Output
The output voltage range of the
MCP6401/1R/1U/2/4/6/7/9 op amps is V
SS
+20mV
(minimum) and V
DD
– 20 mV (maximum) when
R
L
=10kΩ is connected to V
DD
/2 and V
DD
=6.0V.
Refer to Figures 2-26 and 2-27 for more information.
4.3 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. While a unity-gain buffer (G = +1 V/V) 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 V/V), a small series
resistor at the output (R
ISO
in Figure 4-4) 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 capacitance load.
FIGURE 4-4: Output Resistor, R
ISO
Stabilizes Large Capacitive Loads.
Figure 4-5 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., -1 V/V gives G
N
= +2 V/V).
FIGURE 4-5: 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 MCP6401/1R/1U/2/4/6/7/9 SPICE
macro model are very helpful.
4.4 Supply Bypass
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
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 Unused Op Amps
An unused op amp in quad packages (MCP6404 or
MCP6409) should be configured as shown in Figure 4-
6. These circuits prevent the output from toggling and
causing crosstalk. Circuit 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, which buffers that reference
voltage. Circuit B uses the minimum number of
components and operates as a comparator, but it may
draw more current.
FIGURE 4-6: Unused Op Amps.
V
IN
R
ISO
V
OUT
C
L
–
+
MCP640x
1
10
100
1000
10000
1.E-11 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06
Normalized Load Capacitance; C
L
/G
N
(F)
Recommended R
ISO
(Ω)
G
N
:
1 V/V
2 V/V
≥
5 V/V
V
DD
= 6.0 V
R
L
= 10 kΩ
10p 100p 1n 10n 0.1µ 1µ
V
DD
V
DD
R
1
R
2
V
DD
V
REF
V
REF
V
DD
R
2
R
1
R
2
+
-------------------
×
=
¼ MCP6404 (A)
¼ MCP6404 (B)