Stochastic Flash Analog-to-Digital Conversion

A stochastic flash analog-to-digital converter (ADC) is presented. A standard flash uses a resistor string to set individual comparator trip points. A stochastic flash ADC uses random comparator offset to set the trip points. Since the comparators are no longer sized for small offset, they can be shrunk down into digital cells. Using comparators that are implemented as digital cells produces a large variation of comparator offset. Typically, this is considered a disadvantage, but in our case, this large standard deviation of offset is used to set the input signal range. By designing an ADC that is made up entirely of digital cells, it is a natural candidate for a synthesizable ADC. Comparator trip points follow the nonlinear transfer function described by a Gaussian cumulative distribution function, and a technique is presented that reduces this nonlinearity by changing the overall transfer function of the stochastic flash ADC. A test chip is fabricated in 0.18- CMOS to demonstrate the concept.