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The most common implementation of a VCO-based ADC consists of a ring oscillator connected to a thermometer-encoded frequency-to-digital converter. The area of the frequency-to-digital converter depends linearly on the ring oscillator number of taps. In this paper, we show a new frequency-to-digital converter which directly provides a Gray-encoded binary output from the ring oscillator signals. This implementation reduces the digital hardware area in a logarithmic factor keeping the same metastability robustness than the thermometric frequency-to-digital converter. Direct binary coding may simplify data compression and transfer in area-constrained multi-channel instrumentation systems such as neural probes. We show the basic system level structure along with a practical example with circuit simulations in 0.13um CMOS technology.
ring oscillators; voltage-controlled oscillators; frequency conversion; logic gates; thermometers; flip-flops; reflective binary codes