Digital automatic controllers lend themselves to the control of analog variables since they have the following advantages: The time constants can be set digitally, so that temperature dependencies as well as ageing are ruled out. The time constant can be selected arbitrarily large. In this case the use of external components to realize large time constants is dispensed with. As a result, the systems become cost-effective, and the robustness with respect to electromagnetic radiation increases.
FIG. 1 shows such a feedback control loop in the form of a basic block diagram. The digital automatic control of an analog variable requires the use of an analog-digital converter (ADC). It compares its analog input signal, which may be routed via an analog pre-amplifier, with its quantization thresholds and outputs a correspondingly quantized digital value. This digital value is processed by the automatic controller and forwarded to a digital-analog converter, which acts on the analog variable to be controlled by its analog output signal.
However, these feedback control systems have one disadvantage. If the input signal of the ADC is between two quantization thresholds and changes to such a negligible degree that no further quantization threshold is exceeded, then the ADC likewise does not respond to the change of the input signal. That is to say, the analog variable to be regulated may change without the digital automatic controller intervening. Only if the analog signal has changed so significantly that a quantization threshold of the ADC is exceeded, does the digital automatic controller intervene. There is then the risk that the analog variable to be controlled is controlled back and forth between two quantization thresholds of the ADC, which produces a so-called limit cycle. If the frequency of this limit cycle is so small that it is not suppressed by the output filter of the system, then the afore-described effect leads to interference in the output signal of the system.
One possibility for reducing the amplitude of the limit cycle is to reduce the spacing between two quantization thresholds in the ADC, which reduces the quantization error of the ADC. However, with the measuring range remaining unchanged, the number of quantization thresholds must be increased.
Even if the ADC is not used in a feedback control system, there is the risk that the input signal of the ADC changes between two thresholds without the ADC responding by a change in its output signal. This is referred to as a “dead zone”.
Higher demands are imposed on the ADC in the conversion of analog oscillations having a high frequency. To detect the high-frequency analog oscillation, the conversion rate of the ADC must be high. In this case the use of a flash ADC suggests itself, which compares an analog input signal to each of its quantization thresholds at a particular instant. This is accomplished by the use of comparators, each of which compares the input signal to an analog reference.
One possible application example is the automatic control of a high-frequency oscillatory amplitude having a relatively small controller bandwidth. In the process, the analog oscillation is converted into the digital by a flash ADC. The rectification and subsequent deep-pass filtering will then be implemented in the digital realm. This is illustrated in FIG. 2.