A VCO-Based ADC With Inherent Mixing Capability and Local Oscillator Suppression in 55-nm CMOS

This brief introduces a new circuit that can be used as a mixer and as a VCO-ADC simultaneously. The power supply nodes of the inverters in a ring oscillator are sorted in two sets, and connected to two different input signals. We prove mathematically that the oscillator frequency of such configuration reacts to the product of the two analog inputs. Therefore, the ring oscillator can be used as an ADC to directly digitize the low pass down converted component. A first system-level approach is made to minimize the non desired beat components by a proper number of taps in the ring oscillator. To confirm the concept of the Mixer VCO-ADC and its performance, a 55nm CMOS chip has been designed and fabricated. Measurements show a SNDR of 51.3 dB in a 100 kHz bandwidth in the down converted signal from a 1.001 MHz input signal mixed with a 1 MHz local oscillator.

Automated summary

This brief introduces a new circuit that can be used as a mixer and as a VCO-ADC simultaneously. The power supply nodes of the inverters in a ring oscillator are sorted in two sets and connected to two different input signals. The oscillator frequency of such configuration reacts to the product of the two analog inputs, making it possible to directly digitize the low pass down converted component. A system-level approach is made to minimize unwanted beat components by adjusting the number of taps in the ring oscillator. The concept and performance of the Mixer VCO-ADC are confirmed through the design and fabrication of a 55nm CMOS chip, with measurements showing a SNDR of 51.3 dB in a 100 kHz bandwidth.