MCP6V51-ESLASHMS datasheet(25/43 Pages) MICROCHIP

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DS20006136A-page 25

MCP6V51

4.3.7

STABILIZING OUTPUT LOADS

This family of zero-drift op amps has an output

impedance (Figure 2-28 and Figure 2-29) that has a

double zero when the gain is low. This can cause a

large phase shift in feedback networks that have low-

impedance near the part’s cross-over frequency. This

phase shift can cause stability problems.

Figure 4-8 shows that the load on the output is

(RL +RISO)||(RF +RG), where RISO is before the load.

This load needs to be large enough to maintain

stability; it is recommended to design for a total load of

10 kΩ, or higher.

FIGURE 4-8:

Output Resistor, RISO,

Stabilizes Capacitive Loads

4.3.8

GAIN PEAKING

Figure 4-9 shows an op amp circuit that represents

noninverting amplifiers (VM is a DC voltage and VP is

the input) or inverting amplifiers (VP is a DC voltage

and VM is the input). The CN and CG capacitances

represent the total capacitance at the input pins; they

include

the

amp’s

Common

Mode

Input

Capacitance (CCM), board parasitic capacitance and

any capacitor placed in parallel. The CFP capacitance

represents the parasitic capacitance coupling between

the output and the non-inverting input pins.

FIGURE 4-9:

Amplifier with Parasitic

Capacitance.

CG acts in parallel with RG (except for a gain of +1 V/

V), which causes an increase in gain at high

frequencies. CG also reduces the phase margin of the

feedback loop, which becomes less stable. This effect

can be reduced by either reducing CG or RF||RG.

CN and RN form a low-pass filter that affects the signal

at VP. This filter has a single real pole at 1/(2πRNCN).

The largest value of RF that should be used depends

on the noise gain (see GN in Section 4.3.6

“Capacitive Loads”), CG and the open-loop gain’s

phase shift. An approximate limit for RF is shown in

Equation 4-4.

EQUATION 4-4:

Some applications may modify these values to reduce

either output loading or gain peaking (step-response

overshoot).

At high gains, RN needs to be small, in order to prevent

positive feedback and oscillations. Large CN values

can also help.

VOUT

MCP6V51

RISO

VOUT

MCP6V51

CFP

RF 10 k

3.5 pF

--------------- GN2





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