Johnson Counter-Based Multiphase Generation for VCO-Based ADC for Direct Digitization of Low Amplitude Sensor Signals

Voltage-controlled oscillator (VCO)-based analog-to-digital converter (ADC) have been used for the direct digitization of sensor signals. This article proposes a Johnson counter (JC) based multiphase generation for VCO-based ADC for direct digitization of low amplitude sensor signals. The proposed approach reduces the flicker noise of the VCO, thereby giving better signal-to-noise (SNR) ratio. A prototype design is implemented at a printed circuit board (PCB) level using discrete components achieving a signal-to noise + distortion ratio (SNDR) SNDR of 59.2 dB over a 5 kHz bandwidth for
$200 \text {mV}_{\text {pp},\text {diff}}$
input signal using an eight-phase JC-based VCO ADC sampled at 2.5 MHz. The proposed design also achieves a dynamic range of 64 dB for 5-kHz bandwidth and is therefore ideal for many sensor interface applications.