Experimental investigations of floating-gate circuits for Δ - Σ modulators

In this paper we present our initial experimental measurements oil floating-gate structures in Δ-Σ Analog-to-Digital converters. We used floating-gate structures configured as e-pots as part of the modulator loop to provide convenient, and accurate programmable on-chip reference voltages. We use these e-pot outputs as tunable 1-bit DAC coefficients, which could be coupled with the ability to vary integrator gains. This approach is the first application of programmable floating-gate circuits towards Δ-Σ modulators. We see tunable coefficients allowing a designer to realize within limit suitable noise shaping functions, and achieve higher SNR. This approach results in a design of a Δ-Σ ADC that is relatively tolerant to fabrication mismatches. As a result, this approach may improve loop stability and dynamic range of an ADC, particularly for higher order modulators. Finally, using multiple programmable coefficients for several on-chip A DCs results in immense benefits when building multi-channel, higher order modulators. We have fabricated a single loop 1-bit Δ-Σ modulator that is scalable to 8th order, to test these hypotheses. Experimental measurements were obtained from ICs fabricated in 0.5 μm CMOS process available through MOSIS. We use 16 e-pots in the ADC to implement the reference voltages for an 8th order modulator The e-pots are implemented as a regular array structure. The chip operates from a single 5 V supply.