An investigation into the effects of power supply ripple on mixed signal devices and counter measures thereof

Precision instruments using mixed signal devices such as analog to digital converters (ADCs), digital to analog converters (DACs) and frequency synthesisers are sensitive to power supply noise and ripple associated with switch mode power supplies. The spectral composition of noise on a switch mode power supply output is examined. The question is posed as to whether precision mixed signal loads are sensitive to all power supply noise all of the time, or just some of the noise, some of the time The detailed nature of that sensitivity for each converter architecture is explored. Empirical power supply noise sensitivity data measured on a variety of ADC and DAC architectures, amplifiers and frequency synthesisers is presented. The performance degradation of these precision analog components resulting from the presence of power supply noise is shown. New data relating to the nature of a sigma-delta ADC’s sensitivity to supply borne interference is presented. Passive low pass filters that adequately attenuate power supply noise incur a significant space penalty. Linear regulators used as power supply filters require a voltage drop that may be prohibitive. Two novel, different circuit techniques are proposed as solutions. Firstly, a completely novel technique whereby the power supply itself is subjected to a form of track and hold is proposed. That is to take account of the time-discontinuous nature of ADC sensitivity. The sinc function low pass filter characteristic of sampled and held waveforms is beneficially utilized as a power supply ripple filter. This technique is applied to both modified successive approximation and a sigma-delta ADCs. Two different ADC systems were redesigned to incorporate and test the necessary switch. Secondly, parallel resonance as a filter mechanism is proposed. The problems associated with using parallel resonance that have prevented its widespread adoption to date are covered and a novel auto tuning control circuit is presented in response. The automatic tuning circuit uses a specially fabricated barium strontium titanate (BST) variable capacitor as the tuning element. A phase comparison circuit is demonstrated as the control circuit. Results are presented from measurements taken powering a 16 bit pipelined ADC with and without the proposed filter. The volume required to accomplish the auto-tuned BST filter is compared to that achievable with a traditional LC low pass ripple blocking filter. An area saving of over 90% is shown. The unique suitability of this novel solution to full surface mount package encapsulation of the entire regulator and new filter is demonstrated. The work has yielded several patents.