Content area
Full text
A new digital nonlinearity calibration and correction technique is proposed for multi-bit DACs in ΔΖ and incremental modulators. By finding the differences between capacitance errors using the ΔΖ modulator, the integral nonlinearity of the DAC can be found, and corrected digitally.
(ProQuest: ... denotes formulae omitted.)
Introduction: Delta -sigma (DS) modulators with multi-bit feedback DACs have many advantages, such as having better stability and signal-to-quantisation noise ratio (SQNR), than those with single-bit DACs. However, the DAC nonlinearity leads to harmonic distortion of the input signal, and thus reduces the signal-to-noise-and-distortion ratio (SNDR) of the modulator.
The most popular solution to the DAC nonlinearity problem is dataweighted averaging (DWA) [1], a dynamic element matching scheme. It mitigates the effect of DAC nonlinearity by rotating the elements in the DAC, and thus highpass filters the mismatch error. However, it is less effective for high-order DS modulators, and it introduces additional delay in the feedback path, which may be harmful in both discrete and continuous-time delta - sigma modulators. In addition, DWA requires fairly complex digital logic, which may dissipate considerable power during data conversion.
An alternative approach [2, 3] is the digital calibration and correction of the DAC. We can reconfigure the modulator to measure the integral nonlinearity (INL) of the feedback DAC. From the INL, the errors introduced by the DAC can be found. Since the transfer function from the DAC output to modulator output is close to one, one can simply subtract the sequence of errors at the modulator output.
In this Letter, a power-up calibration scheme is proposed which utilises DWA in the DAC during the calibration (but not during conversion) to find and store the mismatch errors for all digital output codes. These can then be simply subtracted from the digital output during conversion. This process is faster and less power hungry than either DWA, or earlier calibration schemes, and eliminates the delay introduced by the DWA during conversion.
DAC error measurement using modulator: A DS modulator can operate as an incremental ADC (IADC) to measure its DC input [4].For example, in a second-order delta - sigma modulator one can process the modulator output v(k) to find the digital...