MCP6141 datasheet(11/32 Pages) MICROCHIP

11 / 32 page

 2002 Microchip Technology Inc.

21668A-page 11

MCP6141/2/3/4

FIGURE 3-2:

Noise gain for inverting and

non-inverting amplifier configuration.

Figure 3-2 shows non-inverting and inverting amplifier

circuits. In order for the amplifiers to be stable, the

noise gain should meet the specified requirement:

EQUATION

Note that an inverting signal gain of G = -9 V/V corre-

sponds to a noise gain Gn = +10 V/V.

Figure 3-3 shows a unity gain buffer and integrator that

are unstable when used with the MCP6141/2/3/4 fam-

ily. However, they are suitable for the MCP6041/2/3/4

family.

FIGURE 3-3:

Typical Circuits that are not

suitable for the MCP6141/2/3/4 family.

Note that the integrator circuit in Figure 3-3 becomes

unity gain at high frequencies because of the capaci-

tor. Therefore, this circuit is unstable for the

MCP6141/2/3/4.

3.6

Capacitive Load and Stability

Driving capacitive loads can cause stability problems

with voltage feedback op amps. Figure 2-21 shows how

increasing the load capacitance will decrease the phase

margin. While a phase margin above 60° is ideal, 45° is

on the verge of instability. As can be seen, up to

CL = 150 pF can be placed on the MCP6141/2/3/4 op

amp outputs without any problems, while 250 pF cre-

ates a 45° phase margin.

When the op amp is required to drive large capacitive

loads (CL >150 pF), a small series resistor (RISO in

Figure 3-4) at the output of the amplifier improves the

phase margin. This resistor makes the output load

resistive at higher frequencies, which improves the

phase margin. The bandwidth reduction caused by the

capacitive load, however, is not changed. To select

RISO, start with 1 kΩ, then use the MCP6141 SPICE

macro model and bench testing to adjust RISO until

there is a minimum frequency response peaking.

FIGURE 3-4:

Amplifier circuit for heavy

capacitive loads.

3.7

The MCP6143 Chip Select (CS)

Option

The MCP6143 is a single amplifier with a chip select

(CS) option. When CS is pulled high, the supply current

drops to 20 pA (typ.) and goes through the CS pin to

VSS. When this happens, the amplifier 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 3-5 shows the output

voltage and supply current response to a CS pulse.

VIN

VOUT

MCP614X

VIN

VOUT

MCP614X

Non-inverting noise gain: 1 + RF/RG ≥ +10 V/V

Inverting noise gain: 1 + RF/RG ≥ +10 V/V

-------

10V/V

≥

VIN

VOUT

VIN

VOUT

MCP604X

Integrator:

Unity gain buffer:

Unstable for MCP614X

VIN

RISO

VOUT

MCP614X

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