The invention relates to an encoder particularly for high speed and high resolution flash analog-to-digital converters (ADC).
Two parts can be distinguished in every flash ADC: an analog and digital sections. The analog section of conventional N-bit flash ADCs consists of a reference source, chain of 2.sup.N -1 resistors and the same number of comparators. The digital section consists of an encoder for sampling comparator output signals, correcting faulty codes and providing binary output code.
The digital output code from the analog section of a flash ADC provided by comparators is referred to as thermometer code since only one pair of adjacent bits is 0 and 1. The code must be sampled and stored as it is unstable due to changes of the ADC input signal, offset voltage of comparators, noise, etc.
The huge number of the comparators in high resolution flash ADCs results in a very complex gate structure of the encoders. Certain kinds of errors caused by a false response of the comparators are not possible to correct due to technological limitations. Simplified error correction schemes result in missing codes. For instance, the flash ADCs have a tendency to miss a code every few billion conversions, i.e. several times a minute when an ADC is sampling at 100MHz.