MCP6H91/2/4
4.2
Rail-to-Rail Output
1000
Recom
mmend R
ISO
(:)
ded
V
DD
= 12 V
R
L
= 10 k
The output voltage range of the MCP6H91/2/4 op amps
is 0.020V (typical) and 11.980V (typical) when
R
L
= 10 k is connected to V
DD
/2 and V
DD
= 12V.
Refer to
Figures 2-24
through
for more
information.
100
4.3
Capacitive Loads
10
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 = +1V/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 = + 1V/V), a small series
resistor at the output (R
ISO
in
improves the
feedback loop’s phase margin (stability) by making the
output load resistive at higher frequencies. The
bandwidth will generally be lower than the bandwidth
with no capacitance load.
G
N
:
1 V/V
2 V/V
t
5 V/V
1
10p
100p
p
1n
10n
0.1μ
μ
1μ
μ
1.E p
1.E-11 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06
11 1.E 10 1.E 09 1.E 08 1.E 07 1.E 06
Normalized Load Capacitance; C
L
/G
N
(F)
FIGURE 4-5:
Recommended R
ISO
Values
for Capacitive Loads.
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
–
V
IN
MCP6H9X
+
R
ISO
V
OUT
C
L
Unused Op Amps
FIGURE 4-4:
Output Resistor, R
ISO
Stabilizes Large Capacitive Loads.
gives the 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., -1V/V gives G
N
= +2V/V).
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 MCP6H91/2/4 SPICE macro
model are helpful.
An unused op amp in a quad package (MCP6H94)
should be configured as shown in
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, and 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.
¼ MCP6H94 (A)
V
DD
R
1
R
2
V
DD
V
REF
¼ MCP6H94 (B)
V
DD
R2
V REF = VDD
--------------------
R1 + R2
FIGURE 4-6:
Unused Op Amps.
DS25138B-page 18
2012 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
