Datasheet

MCP606/7/8/9
DS11177F-page 16 © 2009 Microchip Technology Inc.
4.2 Rail-to-Rail Output
There are two specifications that describe the
output-swing capability of the MCP606/7/8/9 family of
op amps. The first specification (Maximum Output
Voltage Swing) defines the absolute maximum swing
that can be achieved under the specified load
conditions. For instance, the output voltage swings to
within 15 mV of the negative rail with a 25 kΩ load to
V
DD
/2. Figure 2-23 shows how the output voltage is
limited when the input goes beyond the linear region of
operation.
The second specification that describes the output-
swing capability of these amplifiers (Linear Output
Voltage Range) defines the maximum output swing that
can be achieved while the amplifier still operates in its
linear region. To verify linear operation in this range, the
large-signal DC Open-Loop Gain (A
OL
) is measured at
points inside the supply rails. The measurement must
meet the specified A
OL
conditions in the specification
table.
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, though all gains show the
same general behavior.
When driving large capacitive loads with these op
amps (e.g., > 60 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
=+2V/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 simula-
tions with the MCP606/7/8/9 SPICE macro model are
helpful.
4.4 MCP608 Chip Select
The MCP608 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. The CS pin has an internal 5 MΩ (typical)
pull-down resistor connected to V
SS
, so it will go low if
the CS
pins is left floating. Figure 1-1 shows the output
voltage and supply current response to a CS pulse.
4.5 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 also needs 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.
V
IN
R
ISO
V
OUT
C
L
MCP60X
100
1000
10000
10 100 1000 10000
Normalized Load Capacitance; C
L
/G
N
(F)
Recommended R
ISO
(Ω)
10p 10n1n100p
100
10k
1k
G
N
= +1
G
N
= +2
G
N
≥
+4