Bunnie Hwang's "Betrusted" project uses a pair of circuits as an entropy source, with a few blog posts describing the how and why of the design. One source is

Bunnie Hwang's "Betrusted" project uses a pair of circuits as an entropy source, with a few blog posts describing the how and why of the design. One source is a ring oscillator, internal to the Betrusted FPGA. The other is an avalanche noise source similar to OP, external to the FPGA. The two streams are XOR'd and then run through a cryptographic hash. https://github.com/betrusted-io/betrusted-wiki/wiki/TRNG-characterization https://github.com/betrusted-io/betrusted-wiki/wiki/TRNG-cha... https://github.com/betrusted-io/betrusted-wiki/wiki/TRNG-Data-Conditioning https://github.com/betrusted-io/betrusted-wiki/wiki/TRNG-Dat...