AN-1708| Application Note
LMV841 Stability Considerations
LMV841 Stability Considerations
National Semiconductor Application Note 1708 October 2007
Introduction
The following Application Notes apply only for LMV841MG devices with date codes M0730 (units marked with single-digit date code "W") and earlier. For all other devices, please refer to the Application Notes contained in the LMV841/LMV842/ LMV844 Datasheet. For more information, please contact your local National Semiconductor Sales Representative.
Reducing Overshoot
When the output of the op amp is at its lower swing limit (i.e. saturated near V-), rapidly rising signals can cause some overshoot. This overshoot can be reduced by adding a resistor from the output to V+. Even in extreme situations at high temperatures, a 10 k resistor is sufficient to reduce the overshoot to negligible levels. The resistor at the output will however reduce the maximum output swing, as would any resistive load at the output.
Decoupling and Layout
For decoupling the supply lines it is suggested that 10 nF capacitors be placed as close as possible to the op amp. For single supply, place a capacitor between V+ and V-. For dual supplies, place one capacitor between V+ and the board ground, and the second capacitor between ground and V-.
Capacitive Load
The LMV841 can be connected in the voltage-follower configuration; which is most-sensitive to capacitive loading. The combination of a capacitive load placed on the output of an amplifier along with the amplifier's output impedance creates a phase lag, which reduces the phase margin of the amplifier. If the phase margin is significantly reduced, the response will be underdamped which causes peaking in the transfer and when there is too much peaking the op amp might start oscillating. In order to drive heavier capacitive loads, an isolation resistor, RISO, should be used, as shown in Figure 1. By using this isolation resistor, the capacitive load is isolated from the amplifier's output, and hence, the pole caused by CL is no longer in the feedback loop. The larger the value of RISO, the more stable the output voltage will be. If values of RISO are sufficiently large, the feedback loop will be stable, independent of the value of CL. However, larger values of RISO result in reduced output swing and reduced outp
ut current drive.
30036501
TA = 25 C, VS = 5V
FIGURE 2. Small Signal Step Response with Gain = 1
30036502
FIGURE 1. Isolating Capacitive Load
AN-1708
2007 National Semiconductor Corporation
300365
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