Shared Auto-Zero DAC for Photon Counting Comparator Arrays
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Solution Overview
Problem
Conventional electromagnetic radiation sensor applications, such as multi-energy spectral CT, require separate digital-to-analog converters (DACs) for each comparator, consuming significant chip area and power due to tight offset and gain error specifications.
Innovation Solution
Implement a single DAC for multiple comparators using switching circuitry and control logic to individually provide an auto-zero (AZ) reference voltage, allowing each comparator to be disconnected from a common input for auto-zeroing, reducing chip area and power consumption while maintaining operational efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a separate DAC is provided for each comparator, then offset and gain error specifications are met, but chip area and power consumption increase significantly
Solution Approach 1:
Multiple separate DACs for individual comparators are merged into a single shared DAC that serves all comparators. The DAC output is distributed through switching circuitry that connects the single DAC to multiple comparator inputs, thereby reducing chip area while maintaining the required precision through time-multiplexed operation.
Solution Approach 2:
A single DAC is designed to perform the function of multiple DACs by serving all comparators universally. The DAC output can be switched to different comparators in sequence, allowing one component to fulfill the role of several components while maintaining measurement precision through systematic error correction.
2Measurement precision
If a separate DAC is provided for each comparator, then offset and gain error specifications are met, but power consumption increases significantly
Solution Approach 1:
Multiple power-consuming DAC units are merged into a single DAC unit that is shared among all comparators. By consolidating the DAC function, the total power consumption is reduced while the precision requirements are still met through the use of a single high-precision DAC that serves multiple comparators in a time-multiplexed manner.
3Measurement precision
If comparators are disconnected for auto-zeroing, then offset errors are corrected, but dead time increases
Solution Approach 1:
The auto-zeroing process is designed to maintain continuity of useful action by ensuring that while one comparator is disconnected for auto-zeroing, other comparators remain operational and continue processing signals. This overlapping operation minimizes the overall dead time and maintains continuous measurement capability across the system.
Solution Approach 2:
Auto-zeroing is performed periodically rather than continuously, allowing comparators to switch between measurement mode and auto-zero mode in a periodic cycle. This periodic action enables offset correction to be applied at regular intervals without permanently disabling the comparators, thereby reducing the impact on measurement continuity.
Data Source
AI summary
A signal processing arrangement for an electromagnetic radiation sensor application includes a common input, auto-zero, AZ, comparators. Each AZ comparator comprises a signal input for receiving a voltage to be compared and a signal output coupled to a processing block. The processing block evaluates comparison results from the AZ comparators. An AZ DAC provides an AZ reference voltage at an AZ reference output. Switching circuitry individually couples the common input and the AZ reference output to the signal input of each of the AZ comparators in a switchable fashion. A control block disconnects, by controlling the switching circuitry, for each of the AZ comparators in a non-overlapping fashion, the common input from the signal input of the respective AZ comparator during a respective disconnection time, and connect the AZ reference output within the respective disconnection time to the signal input of the respective AZ comparator for a respective AZ time.