OPA336-EP Datasheet, PDF (9/15 Pages) Texas Instruments – SINGLE-SUPPLY MICRO-POWER CMOS OPERATIONAL AMPLIFIER MicroAmplifier™ SERIES

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OPA336-EP
SINGLE-SUPPLY MICRO-POWER CMOS OPERATIONAL AMPLIFIER
MicroAmplifier™ SERIES
SCES658 – JUNE 2006
APPLICATION INFORMATION (CONTINUED)
Capacitive Load and Stability
The OPA336 can drive a wide range of capacitive loads. However, all operational amplifiers, under certain
conditions, may become unstable. Operational amplifier configuration, gain, and load value are just a few of the
factors to consider when determining stability.
When properly configured, the OPA336 drives approximately 10,000 pF. An operational amplifier in unity-gain
configuration is the most vulnerable to capacitive load. The capacitive load reacts with the operational amplifier
output resistance along with any additional load resistance to create a pole in the response, which degrades the
phase margin. In unity gain, the OPA336 performs well with a pure capacitive load, up to about 300 pF.
Increasing gain enhances the amplifier's ability to drive loads beyond this level.
One method of improving capacitive load drive in the unity-gain configuration is to insert a 50-Ω to 100-Ω
resistor inside the feedback loop (see Figure 3). This reduces ringing with large capacitive loads, while
maintaining direct current (DC) accuracy. For example, with R_L = 25 kΩ, OPA336 performs well with capacitive
loads in excess of 1000 pF (see Figure 4). Without the OPA336 R_S, capacitive load drive typically is 350 pF for
these conditions (see Figure 5).
R_S
100 Ω
V_OUT
V_IN
C_L
R_L
Figure 3. Series Resistor in Unity-Gain Configuration
Improves Capacitive Load Drive
R_S = 100 Ω, Load = 2 kΩ || 1000 pF, V_s +5 V
50 µs/div
Figure 4. Small-Signal Step Response
Using Series Resistor to Improve Capacitive Load Drive
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