MCP6V06 Datasheet, PDF (25/44 Pages) Microchip Technology – 300 μA, Auto-Zeroed Op Amps

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 MCP6V06 SPICE macro model
(good for all of the MCP6V06/7/8 op amps) are helpful.
4.3.6 STABILIZING OUTPUT LOADS
This family of auto-zeroed op amps has an output
impedance (Figure 2-31 and Figure 2-32) that has a
double zero when the gain is low. This can cause a
large phase shift in feedback networks that have low
resistance near the part’s bandwidth. This large phase
shift can cause stability problems.
Figure 4-8 shows one circuit example that has low
resistance near the part’s bandwidth. R_F and C_F set a
pole at 0.16 kHz, so the noise gain (G_N) is 1 V/V at the
circuit’s bandwidth (roughly 1.3 MHz). The load seen
by the op amp’s output at 1.3 MHz is R_G||R_L (99Ω).
This is low enough to be a real concern.
V_IN
R_N
100Ω
MCP6V0X
R_G
100Ω
R_F
10.0 kΩ
V_OUT
R_L
10.0 kΩ
C_F
0.1 µF
FIGURE 4-8:
Output Load Issue.
To solve this problem, increase the resistive load to at
least 3 kΩ. Methods to accomplish this task include:
• Increase R_G
• Remove C_F (relocate the filter)
• Add a 3 kΩ resistor at the op amp’s output that is
not in the signal path; see Figure 4-9
V_IN
R_N
100 Ω
R_G
100 Ω
FIGURE 4-9:
Load Issue.
MCP6V0X
R_F
10.0 kΩ
R_X
3.01 kΩ
V_OUT
R_L
10.0 kΩ
C_F
0.1 µF
One Solution To Output
MCP6V06/7/8
4.3.7
REDUCING UNDESIRED NOISE
AND SIGNALS
Reduce undesired noise and signals with:
• Low bandwidth signal filters:
- Minimizes random analog noise
- Reduces interfering signals
• Good PCB layout techniques:
- Minimizes crosstalk
- Minimizes parasitic capacitances and induc-
tances that interact with fast switching edges
• Good power supply design:
- Isolation from other parts
- Filtering of interference on supply line(s)
4.3.8
SUPPLY BYPASSING AND
FILTERING
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
of the pin for good high-frequency performance.
These parts also need 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 low noise,
analog parts.
Additional filtering of high frequency power supply
noise (e.g., switched mode power supplies) can be
achieved using resistors. The resistors need to be
small enough to prevent a large drop in V_DD for the op
amp, which would cause a reduced output range and
possible load-induced power supply noise. The resis-
tors also need to be large enough to dissipate little
power when V_DD is turned on and off quickly. The cir-
cuit in Figure 4-10 gives good rejection out to 1 MHz for
switched mode power supplies. Smaller resistors and
capacitors are a better choice for designs where the
power supply is reasonably quiet.
V_{S_ANA}
143Ω
1/4W
143Ω
1/10W
100 µF 100 µF
to other analog parts
0.1 µF
MCP6V0X
FIGURE 4-10:
Additional Supply Filtering.
© 2008 Microchip Technology Inc.
DS22093B-page 25