15.2 A 2.2GHz continuous-time ΔΣ ADC with −102dBc THD and 25MHz BW

Abstract

The trend towards wideband radio front-ends (RFE) for car radio receivers allows for digital tuning and channel filtering, simultaneous reception of multiple channels and reduced interference. A filter-less RFE requires a high dynamic range and very linear analog-to-digital converter (ADC). For AM/FM, typical total harmonic distortion (THD), intermodulation distortion and spurious tone levels of the ADC must be below −100dBc. These nonlinearity levels are much lower than the required ADC noise that is necessary to maintain high reception quality of a weak wanted signal in the presence of strong unfiltered interferers. Such audio-like linearity performance has been demonstrated for low input signal frequencies up to several hundreds of kHz [1–2], but the spurious free dynamic range (SFDR) of ADCs with >10MHz bandwidth is typically limited to 80 to 90dB [3–4] due to static mismatches associated with multi-bit quantization and dynamic errors such as data-dependent disturbances on the DAC supply [1]. This paper presents a continuous-time (CT) ΔΣ ADC that achieves −102dB THD, −104dB IM3, >110dB SFDR and 77dB SNDR in 25MHz bandwidth over process, voltage and temperature (PVT) variations. Such high linearity is achieved by using a 1b feedback DAC, which is highly insensitive to process spread and mismatch, in combination with a wideband high-precision voltage regulator that is designed to mitigate dynamic errors of the 1b DAC.