MCP6V06T datasheet(25/44 Pages) MICROCHIP

25 / 44 page

DS22093B-page 25

MCP6V06/7/8

After selecting RISO for your circuit, double check the

resulting frequency response peaking and step

response overshoot. Modify RISO'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. RF and CF set a

pole at 0.16 kHz, so the noise gain (GN) 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 RG||RL (99Ω).

This is low enough to be a real concern.

FIGURE 4-8:

Output Load Issue.

To solve this problem, increase the resistive load to at

least 3 k

Ω. Methods to accomplish this task include:

• Increase RG

• Remove CF (relocate the filter)

• Add a 3 k

Ω resistor at the op amp’s output that is

not in the signal path; see Figure 4-9

FIGURE 4-9:

One Solution To Output

Load Issue.

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 (VDD 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 VDD 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 VDD 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.

FIGURE 4-10:

Additional Supply Filtering.

MCP6V0X

10.0 k

100

VOUT

VIN

10.0 k

0.1 µF

MCP6V0X

10.0 k

100

VOUT

VIN

10.0 k

0.1 µF

3.01 k

MCP6V0X

VS_ANA

143

100 µF

0.1 µF

1/4W

1/10W

to other analog parts

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