MCP6H71 Datasheet, PDF (18/44 Pages) –

MCP6H71/2/4
4.2 Rail-to-Rail Output
The output voltage range of the MCP6H71/2/4 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 2-29 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 = +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 Figure 4-4) 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.
–
R_ISO
MCP6H7X
V_IN
+
C_L
V_OUT
FIGURE 4-4:
Output Resistor, R_ISO
Stabilizes Large Capacitive Loads.
Figure 4-5 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 MCP6H71/2/4 SPICE macro
model are helpful.
1000
100
V_DD = 12 V
R_L = 10 kȍ
G_N:
10
1 V/V
2 V/V
t 5 V/V
1
1.1E0-p11 11.E00-1p0 1.E1-n09 1.E1-00n8 1.E0-.017µ 1.E1-0µ6
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 Unused Op Amps
An unused op amp in a quad package (MCP6H74)
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, 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 MCP6H74 (A)
V_DD
R₁
V_DD
R₂
¼ MCP6H74 (B)
V_DD
V_REF
VREF
=
VDD

--------R---2---------
R1 + R2
FIGURE 4-6:
Unused Op Amps.
DS20002325C-page 18
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