LMV821 datasheet(13/26 Pages) TI1

13 / 26 page

Application Note (Continued)

UNITY GAIN PULSE RESPONSE CONSIDERATION

A pull-up resistor is well suited for increasing unity-gain,

pulse response stability. For example, a 600

Ω pull-up resis-

tor reduces the overshoot voltage by about 50%, when

driving a 220 pF load. Figure 3 shows how to implement the

pull-up resistor for more pulse response stability.

Higher capacitances can be driven by decreasing the value

of the pull-up resistor, but its value shouldn’t be reduced

beyond the sinking capability of the part. An alternate ap-

proach is to use an isolation resistor as illustrated in Figure 4

Figure 5 shows the resulting pulse response from a LMV824,

while driving a 10,000 pF load through a 20

Ω isolation

resistor.

INPUT BIAS CURRENT CONSIDERATION

Input bias current (I

B) can develop a somewhat significant

offset voltage. This offset is primarily due to I

flowing

through the negative feedback resistor, R

F. For example, if IB

is 90 nA (max @ room) and R

F is 100 k

Ω, then an offset of 9

mV will be developed (V

OS=IBxRF).Using a compensation

resistor (R

C), as shown in Figure 6, cancels out this affect.

But the input offset current (I

OS) will still contribute to an

offset voltage in the same manner - typically 0.05 mV at

room temp.

APPLICATION CIRCUITS

This section covers the following application circuits:

1. Telephone-Line Transceiver

2. “Simple” Mixer (Amplitude Modulator)

10012861

FIGURE 2. Unity-Gain Frequency vs Common Mode

Voltage for Various Loads

10012841

FIGURE 3. Using a Pull-up Resistor at the Output for

Stabilizing Capacitive Loads

10012843

FIGURE 4. Using an Isolation Resistor to Drive Heavy

Capacitive Loads

10012854

FIGURE 5. Pulse Response per Figure 4

10012859

FIGURE 6. Canceling the Voltage Offset Effect of Input

Bias Current

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