Front-end electronics for capacitive sensors typically include a preamplifier followed by a filter. The preamplifier provides low-noise amplification of the signals induced in the sensor electrodes. The filter, by properly limiting the signal bandwidth, maximizes the Signal-to-Noise (S/N) ratio. Additionally, the filter limits the duration of the output signal associated with the measured event and, for those sensors where the induced signal is relatively slow, it maximizes the signal amplitude.
Filters can be either time-variant or time-invariant. In electronics for radiation sensors, time-invariant filters are frequently referred to as “shapers” since, in a timedomain view, they shape the response associated with events. Filters can also be synthesized digitally, even though in most cases, this is impractical due to constraints from power and real-estate budgets.
An application-specific integrated circuit (ASIC) was developed that implements semi-Gaussian filters (pulse shapers) to detect signals with low noise and high resolution. Compared to other devices, this ASIC for Delayed Dissipative Feedback (DDF) achieves higher dynamic range at comparable area and equal capacitance, improving the S/N ratio. Conventional designs based on voltage amplifiers with local feedbacks introduce noise that limits the dynamic range, but this novel design overcomes that obstacle.
The circuit delays, wherever possible, the feedback of the resistive (dissipative) components. The negative dissipative feedback through at least one resistor to an input of an amplifier of the shaper is provided from a voltage output by a subsequent circuit stage, and is delayed by one or more time constants. The design of low-noise linear shapers suggests that once the equivalent noise charge (ENC) from the charge amplifier is defined, then the dynamic range of the system is set by the voltage swing and the value of the capacitance to achieve the poles of the shaper.
The configuration used to generate the pole(s) also has relevant impact. Configurations based on passive components in feedback offer a better dynamic range than those using both active and passive components, such as scaling mirrors.
Delayed Dissipative Feedback can help overcome some of the limitations of the more classical ASIC configurations. For example, in a third-order shaper, dynamic range may be increased by a factor of two, diminishing noise and increasing signal clarity.
In an alternative embodiment, the input node of the first filter stage in the chain of filter stages may be coupled to a current source. The output at the output node of the last filter stage may be voltage.