Effect of a PWM feedback DAC on the noise and linearity of a delta-sigma ADC

In the development of a sub ppm, metrology class, delta-sigma ADC, pulse width modulation (PWM) was chosen to achieve multi-bit feedback. PWM DACs have been used in precision DC calibrators operating with linearity in the 1E-7 region and it is well known that multi-bit DACs in delta sigma modulators offer significant advantages over simple 1 bit types. This paper describes many of the practical difficulties encountered and overcome in combining these two techniques to achieve metrology class performance. This work continues from ideas presented at ADDA Pickering, JR (2002) following on from pioneering work reported at ADDA Pett, JG (1999). For a general treatise on delta-sigma see "Candy & Temes" (1992).