Differential Offset Calibration in Time-Interleaved ADCs
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Solution Overview
Problem
Time-interleaved analog-to-digital converters (ADCs) face limitations in offset calibration when sampling signals at multiples of the channel sampling frequency, leading to signal aliasing and removal by offset calibration blocks, which restricts the acquisition of input signals and introduces flicker noise and residual spurs.
Innovation Solution
The implementation of a differential offset calibration technique using a plurality of channels with a switch, offset calibration circuits, a multiplier, and a pseudorandom bit sequence (PRBS) generator, along with gain and time-skew calibration circuits, allows for the removal of time-interleaved offset spurs and enables signal acquisition at multiples of the sampling frequency without signal corruption, using a method that disables and enables components to manage offset and flicker noise effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If offset calibration blocks are used to remove offset in time-interleaved ADCs, then offset removal is achieved, but signals at multiples of the sampling frequency are aliased to DC and removed completely
Solution Approach 1:
The system performs preliminary offset calibration by disabling the differential input and operating the offset calibration circuits before enabling the normal input signal. This preliminary action removes offset without interfering with subsequent signal processing, allowing signals at multiples of sampling frequency to be preserved.
Solution Approach 2:
The calibration process uses periodic disabling and enabling of components: the differential input is disabled during offset calibration, then enabled for normal operation. The accumulator is disabled at specific times to prevent integration of calibration signals into the output. This periodic action separates calibration functions from signal processing functions.
2Measurement precision
If conventional offset calibration is used, then offset removal is achieved, but flicker noise and residual spurs are introduced
Solution Approach 1:
The harmful effects (flicker noise and residual spurs) are extracted and separated from the main signal path by using a dedicated calibration mode. During calibration, the differential input is disabled and calibration circuits operate independently, preventing the generation of flicker noise and residual spurs that would otherwise contaminate the signal.
Solution Approach 2:
The system uses an intermediary calibration mode that acts as a mediator between offset removal and signal processing. By disabling the differential input during calibration and using separate calibration circuits, the system eliminates the direct interaction that causes flicker noise and residual spurs while still achieving offset removal.
3Measurement precision
If offset calibration blocks operate continuously, then offset removal is maintained, but the behavior of calibration blocks is affected by input signals
Solution Approach 1:
Offset calibration is performed as a preliminary action before normal signal processing begins. The calibration circuits operate independently with the differential input disabled, establishing stable calibration parameters without being influenced by subsequent input signals. This ensures reliable and consistent calibration block behavior.
Solution Approach 2:
The system segments the operation into distinct phases: calibration phase (with differential input disabled) and signal processing phase (with differential input enabled). This segmentation prevents input signals from affecting calibration block behavior while maintaining offset removal accuracy throughout operation.
Data Source
AI summary
An example apparatus for analog-to-digital conversion includes a plurality of channels each including an analog-to-digital converter (ADC), a switch configured to couple a differential input to the ADC, a first offset calibration circuit coupled to an output of the ADC, a multiplier coupled to an output of the first offset calibration circuit, a second offset calibration circuit coupled to an output of the multiplier, and a pseudorandom bit sequence (PRBS) generator coupled to the switch and the multiplier. The apparatus further includes a gain calibration circuit coupled to an output of the second offset calibration circuit in each of the plurality of channels; and a time-skew calibration circuit coupled to an output of the gain calibration circuit.