LMV821 datasheet(12/26 Pages) TI1

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Typical Performance Characteristics Unless otherwise specified, V

S = +5V, single supply,

TA = 25˚C. (Continued)

THD vs. Frequency

10012882

Application Note

This application note is divided into two sections: design

considerations and Application Circuits.

DESIGN CONSIDERATIONS

This section covers the following design considerations:

1. Frequency and Phase Response Considerations

2. Unity-Gain Pulse Response Considerations

3. Input Bias Current Considerations

FREQUENCY AND PHASE RESPONSE

CONSIDERATIONS

The relationship between open-loop frequency response

and open-loop phase response determines the closed-loop

stability performance (negative feedback). The open-loop

phase response causes the feedback signal to shift towards

becoming positive feedback, thus becoming unstable. The

further the output phase angle is from the input phase angle,

the more stable the negative feedback will operate. Phase

Margin (

m) specifies this output-to-input phase relationship

at the unity-gain crossover point. Zero degrees of phase-

margin means that the input and output are completely in

phase with each other and will sustain oscillation at the

unity-gain frequency.

The AC tables show

m for a no load condition. But

changes with load. The Gain and Phase margin vs Fre-

quency plots in the curve section can be used to graphically

determine the

m for various loaded conditions. To do this,

examine the phase angle portion of the plot, find the phase

margin point at the unity-gain frequency, and determine how

far this point is from zero degree of phase-margin. The larger

the phase-margin, the more stable the circuit operation.

The bandwidth is also affected by load. The graphs of Figure

1 and Figure 2 provide a quick look at how various loads

affect the

m and the bandwidth of the LMV821/822/824

family. These graphs show capacitive loads reducing both

m and bandwidth, while resistive loads reduce the band-

width but increase the

m. Notice how a 600

Ω resistor can be

added in parallel with 220 picofarads capacitance, to in-

crease the

m 20˚(approx.), but at the price of about a 100

kHz of bandwidth.

Overall, the LMV821/822/824 family provides good stability

for loaded condition.

10012860

FIGURE 1. Phase Margin vs Common Mode Voltage for

Various Loads

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