Datasheet

MCP6241/1R/1U/2/4
DS21882D-page 12 © 2008 Microchip Technology Inc.
A significant amount of current can flow out of the
inputs when the common mode voltage (V
CM
) is below
ground (V
SS
); see Figure 2-19. Applications that are
high impedance may need to limit the useable voltage
range.
4.1.3 NORMAL OPERATION
The input stage of the MCP6241/1R/1U/2/4 op amps
use two differential CMOS input stages in parallel. One
operates at low common mode input voltage (V
CM
),
while the other operates at high V
CM
. WIth this topol-
ogy, the device operates with V
CM
up to 0.3V above
V
DD
and 0.3V below V
SS
.
4.2 Rail-to-Rail Output
The output voltage range of the MCP6241/1R/1U/2/4
op amps is V
DD
–35mV (maximum) and V
SS
+35mV
(minimum) when R
L
=10kΩ is connected to V
DD
/2 and
V
DD
= 5.5V. Refer to Figure 2-14 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. A unity-gain buffer (G = +1) is the most
sensitive to capacitive loads, but all gains show the
same general behavior.
When driving large capacitive loads with these op
amps (e.g., > 70 pF when G = +1), 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 capacitive 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. Evaluation on the bench and
simulations with the MCP6241/1R/1U/2/4 SPICE
macro model are very helpful. Modify R
ISO
’s value until
the response is reasonable.
4.4 Supply Bypass
With this op amp, 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 nearby analog parts.
4.5 Unused Op Amps
An unused op amp in a quad package (MCP6244)
should be configured as shown in Figure 4-6. Both
circuits prevent the output from toggling and causing
crosstalk. Circuit A can use any reference voltage
between the supplies, provides a buffered DC voltage,
and minimizes the supply current draw of the unused
op amp. Circuit B minimizes the number of
components, but may draw a little more supply current
for the unused op amp.
FIGURE 4-6: Unused Op Amps.
V
IN
R
ISO
V
OUT
MCP624X
C
L
–
+
1.E+02
1.E+03
1.E+04
1.E+01 1.E+02 1.E+03 1.E+04
Normalized Load Capacitance; C
L
/G
N
(F)
Recommended R
ISO
(Ω)
10p 100p 1n 10n
10k
1k
100
G
N
= +1 V/V
G
N
≥
+2 V/V
V
DD
V
DD
¼ MCP6244 (A) ¼ MCP6244 (B)
R
1
R
2
V
DD
V
REF
V
REF
V
DD
R
2
R
1
R
2
+
------------------
⋅=