Dpo7254 signal path compensation error5/16/2023 ![]() CL open-circuited.īy equating the pole in Case 1 to the zero in Case 2, and the pole in Case 2 to the zero in Case 1, we derive the following two equations: With C L open, the pole and zero are a function of C f. With C f shorted, R x << R f, and R o << R in, the pole and zero are functions of C L, R o, and R x. With this in mind, and referring to the circuit in Figure 4, let’s apply this principle to one capacitor at a time. Think of the capacitors, C f and C L, as open circuits at dc, and shorts at high frequencies. VB is connected to the amplifier’s minus input. To better understand this technique, consider the redrawn feedback portion of the circuit shown in Figure 4. A small series resistor, R x, is used to decouple the amplifier output from C L and a small capacitor, C f, inserted in the feedback loop, provides a high frequency bypass around C L. In-the-loop compensation circuit.įigure 3 shows a commonly used compensation technique, often dubbed in-the-loop compensation. For example, here’s a compensation technique that has the added benefit of filtering the op amp’s noise via an RC feedback circuit. Q: So, different circuits call for different techniques?Ī: Yes, absolutely! You’ll choose the compensation technique that best suits your design. A simple op amp circuit with capacitive load. This note discusses typical questions about the effects of capacitive loads on the performance of some amplifier circuits, and suggests techniques to solve the instability problems they raise. The –20 dB/decade slope and 90° lag contributed by the pole, added to the –20 dB slope and 90° contributed by the amplifier (plus any other existing lags), results in an increase in the rate of closure (ROC) to a value of at least 40 dB per decade, which, in turn, causes instability. The loaded gain can be expressed as follows:Īnd A is the unloaded open-loop gain of the amplifier. Typical applications include sample-and-hold amplifiers, peak detectors, and driving unterminated coaxial cables.Ĭapacitive loading, as shown in Figures 1 and 2, affects the open-loop gain in the same way, regardless of whether the active input is at the noninverting or the inverting terminal: the load capacitance, C L, forms a pole with the open-loop output resistance, R O. To avoid sacrificing performance with light loads, most amplifiers are not heavily compensated internally for substantial capacitive loads, so external compensation techniques must be used to optimize those applications in which a large capacitive load at the output of the op amp must be handled. Ideally, an otherwise stable op amp with R O = 0 will drive any capacitive load without phase degradation. attenuation and phase shift of the feedback circuit, including the effects of output loads, input impedances, and stray capacitances.Īmong the parameters cited above, the amplifier output impedance, represented by the output resistance, R O, is the one factor that most affects performance with capacitive loads.the amplifier’s internal architecture (for example, output impedance, gain and phase margin, internal compensation circuitry).The ability of an op amp to drive capacitive loads is affected by several factors: The problem is especially severe when large capacitive loads, such as LCD panels or poorly terminated coaxial cables, must be driven-but unpleasant surprises in precision low-frequency and dc applications can result as well.Īs will be seen, the op amp is most prone to instability when it is configured as a unity-gain follower, either because (a) there is no attenuation in the loop, or (b) large common-mode swings, though not substantially affecting accuracy of the signal gain, can modulate the loop gain into unstable regions. Although some capacitive loading is inevitable, amplifiers are often subjected to sufficient capacitive loading to cause overshoots, ringing, and even oscillation. But, I need a refresher-NOW.Ĭapacitive loads often give rise to problems, in part because they can reduce the output bandwidth and slew rate, but mainly because the phase lag they produce in the op amp’s feedback loop can cause instability. Q: ADI has published a lot of information on dealing with capacitive loading and other stability issues in books, such as the amplifier seminar series, in earlier issues of Analog Dialogue, and in some design tools. ![]() Ask The Application Engineer-32: Practical Techniques to Avoid Instability Due to Capacitive Loading
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