MCP6141-ESLASHMS datasheet(17/38 Pages) MICROCHIP

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DS21668D-page 17

MCP6141/2/3/4

Figure 4-5 shows three example circuits that are

unstable when used with the MCP6141/2/3/4 family.

The unity gain buffer and low gain amplifier

(non-inverting or inverting) are at gains that are too low

for stability (see Equation 4-2).The Miller integrator’s

capacitor makes it reach unity gain at high frequencies,

causing instability.

FIGURE 4-5:

Examples of Unstable

Circuits for the MCP6141/2/3/4 Family.

4.4.2

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 = +10), a small series

resistor at the output (RISO in Figure 4-6) 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 4-6:

Output Resistor, RISO

stabilizes large capacitive loads.

Figure 4-7 gives recommended RISO values for

different capacitive loads and gains. The x-axis is the

normalized load capacitance (CL/GN), where GN is the

circuit’s noise gain. For non-inverting gains, GN and the

Signal Gain are equal. For inverting gains, GN is

1 + |Signal Gain| (e.g., -9 V/V gives GN = +10 V/V).

FIGURE 4-7:

Recommended RISO Values

for Capacitive Loads.

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 MCP6141/2/3/4 SPICE macro

model are helpful.

4.5

MCP6143 Chip Select

The MCP6143 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 VSS.

When this happens, the amplifier output is put into a

high impedance state. By pulling CS low, the amplifier

is enabled. If the CS pin is left floating, the amplifier will

not operate properly. Figure 1-1 shows the output

voltage and supply current response to a CS pulse.

Note:

The three circuits shown in Figure 4-5 are

not

to be used with the MCP6141/2/3/4 op

amps. They are included for illustrative

purposes only.

Low Gain Amplifier

MCP614X

VOUT

-------

MCP614X

VOUT

VIN

MCP614X

VOUT

VIN

Unity Gain Buffer

Miller Integrator

MCP614X

RISO

VOUT

1,000

10,000

100,000

1.E+00

1.E+01

1.E+02

1.E+03

Normalized Load Capacitance; CL/GN (F)

100k

10p

GN = +10

GN = +20

GN ≥ +50

10k

100p

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