Ever wondered why we need to learn how to drive an operational amplifier (op amp) with a capacitive load? In most cases, the load capacitance is not from a capacitor we've intentionally added. Instead, it often originates from the op amp’s next stage, such as the capacitance introduced at an Analog-to-Digital Converter’s (ADC) input. It can also come from unintended parasitic capacitances, such as those introduced by cables, PCB traces, or even measurement tools like oscilloscope probes.
These capacitances can make the op amp unstable, leading to oscillations on its output. Such oscillations, especially if not compensated, can damage not only the op amp but also the subsequent stages in the signal chain and, thus, the electronic instrument or equipment itself.
As discussed in Part 1 of the Amplifier Stability Essentials blog series, the EVAL-KW4503Z uses the other channel of the ADA4510-2 for the capacitive load stability KWIK demo. This demo allows the user to stabilize a unity-gain op amp (buffer) driving a capacitive load using a compensation resistor, which can be configured via J2. This will enable users to learn how to eliminate unwanted oscillations in the output voltage signal while ensuring that the op amp and the subsequent stages in the signal chain remain protected from damage.
Figure 1. Capacitive Load Stability KWIK Demo
Note: The ADALM2000 Scopy Configuration file for this demo can be found on Stability KWIK Lecture + Lab (Wiki)
Capacitive Load
A capacitive load will react with the output impedance (Zo) of an op amp and will add a pole on the Open-Loop Gain (Aol) curve. As shown in the figure below, the slope of the Aol curve changed to -40dB/dec due to the pole caused by the capacitive load. In addition, on the point of intersection of the Aol curve and 1/β curve (Ideal-Closed Loop Gain), a 40dB/dec rate of closure can be observed, which indicates instability.
Figure 2. Effect of Driving a Capacitive Load
To better understand this, the stability of ADA4510 in a unity-gain buffer configuration, with and without a 1nF capacitive load, was simulated in LTSpice.
Figure 3. Test Circuit for Simulating the Open-Loop Gain and 1/β curve (Ideal-Closed Loop Gain) of ADA4510, with and without a Capacitive Load.
As shown in Figure 4, when a capacitive load is introduced, a pole occurred, which caused the slope of the Aol curve to change to -40dB/dec. A 40dB/dec rate of closure was also observed between the Aol and 1/β curve, indicating instability.
Figure 4. Simulating the Stability of ADA4510 in a Unity-Gain Buffer Configuration, with and without a 1nF Capacitive Load
To visualize the actual transient response, EVAL-KW4503Z along with ADALM2000 was used. By loading the Scopy Configuration file for this demo and installing the jumper on J2 on the demo board, the result below can be obtained:
Figure 5. Transient Response of ADA4510 in a Unity-Gain Buffer Configuration with a 1nF Capacitive Load.
To compensate for this instability, a zero can be added on the Aol curve using a compensation resistor (Rs) connected in between the op amp’s output and the capacitive load. Because of the resistance introduced by Rs, the slope of the Aol curve can return to -20dB/dec and a 20db/dec rate of closure can be achieved at the intersection of the Aol curve and 1/β curve, indicating a Phase Margin of ≥ 45°.
Figure 6. Effect of Adding a Resistor between the Op Amp’s Output and the Capacitive Load
To determine the correct value of the compensation resistor that produces a 20dB/decade rate of closure, different resistor values were simulated in LTSpice. At Rs = 100 Ω, it can now be seen clearly that a zero has been added on the Aol curve, resulting in a 20dB/dec rate of closure relative to the 1/β curve.
Figure 7. Simulating the Stability of ADA4510 in a Unity-Gain Buffer Configuration driving a 1nF Capacitive Load – With Rs = 100Ω
Using the EVAL-KW4503Z together with ADALM2000 again, we can observe the effect of adding Rs on the actual transient response. By loading the same Scopy Configuration file and removing the jumper on J2 on the demo board, the result in Figure 8 can be obtained:
Figure 8. Transient Response of ADA4510 in a Unity-Gain Buffer Configuration with Rs = 100Ω and CL = 1nF
The Bottom Line
Load capacitance, as seen on an op amp’s output, often originates from its next stage, such as the capacitance introduced at an Analog-to-Digital Converter’s (ADC) input, or from unintended parasitic capacitances. These capacitances, if not properly compensated, can make the op amp unstable, leading to unwanted oscillations.
To prevent op amp instability or the unwanted output oscillations caused by a capacitive load, a compensation resistor can be added between the op amp’s output and the load capacitance. This will eliminate the unwanted oscillations in the output voltage signal while ensuring that the op amp and the subsequent stages in the signal chain remain protected from damage.
This concludes the blog series. Always remember, “If there’s instability, don’t forget to compensate!”
Read all the blogs in the Amplifier Stability Essentials series.