LMV921 datasheet(17/23 Pages) TI1

17 / 23 page

OBSOLETE

LMV921, LMV922, LMV924

www.ti.com

SNOS436H – AUGUST 2000 – REVISED APRIL 2013

APPLICATION NOTE

Unity Gain Pulse Response Considerations

The unity-gain follower is the most sensitive configuration to capacitive loading. The LMV921/LMV922/LMV924

family can directly drive 1nF in a unity-gain with minimal ringing. Direct capacitive loading reduces the phase

margin of the amplifier. The combination of the amplifier's output impedance and the capacitive load induces

phase lag. This results in either an underdamped pulse response or oscillation. The pulse response can be

improved by adding a pull up resistor as shown in Figure 52

Figure 52. Using a Pull-Up Resistor at the Output for Stabilizing Capacitive Loads

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 approach is to use an isolation resistor as

illustrated in Figure 53.

Figure 53. Using an Isolation Resistor to Drive Heavy Capacitive Loads

Input Bias Current Consideration

The LMV921/LMV922/LMV924 family has a bipolar input stage. The typical input bias current (IB) is 12nA. The

input bias current can develop a significant offset voltage. This offset is primarily due to IB flowing through the

negative feedback resistor, RF. For example, if IB is 50nA (max room) and RF is 100kΩ, then an offset voltage of

5mV will develop (VOS = IBX RF). Using a compensation resistor (RC), as shown in Figure 54, cancels this affect.

But the input offset current (IOS) will still contribute to an offset voltage in the same manner.

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LMV921 Datasheet(PDF) 17 Page - Texas Instruments