Noise Analysis of the Pulse-Width Modulator for Quasi-Digital Sensors

Majority of available integrated and smart sensors are based on quasi-digitalization concept. A pulse modulation of the analog sensor output signal can be accomplished by using a fairly simple relaxation oscillator circuit. Pulse-width modulation (PWM) is commonly used in such applications, due to its ratiometric output that greatly reduces the error caused by the changes in the time constant of the oscillator. However, most of the literature that deal with the pulse-width modulators do not include the analysis of noise effects, which is important factor in optimizing the readout of PWM signal. In this article, we analyze the effects of noise in the simple dual-slope pulse-width modulator and provide an accurate noise model that can easily be adopted to any other type of pulse-width modulators. The model can be used to predict the jitter of the edges of output PWM signal and to analyze the effects of that jitter on the uncertainty of the measured duty-cycle for various PWM readout techniques.