Design considerations for current domain regenerative comparators

In voltage domain applications, regenerative comparators are a common choice where high speed operation is required. Their basic structure is simple, consisting of pairs of cross coupled devices in a positive feedback or regenerative configuration. However, such structures are notoriously sensitive to offsets and hence when precise operation is required they usually form the second (or later) stage in a multi-stage comparator. The first stage, normally a differential amplifier, ensures that a large difference signal drives the regenerative stage thus giving reliable operation. In high speed sampled current domain applications, a high gain pre-amplification stage is unlikely to be appropriate, resulting in increased accuracy demands on the current driven regenerative latch. In this paper we study the performance of the regenerative comparator under current excitation. The perfectly balanced case is first considered and the performance parameters, which dictate response time for a specified differential input signal magnitude, are identified. The analysis is then repeated for the general asymmetric case and it is shown that both static and dynamic errors further limit the maximum resolution. The analytical equations derived are confirmed with SPICE simulations. Finally, a system application is presented, a sigma-delta modulator for direct digitisation of photodiode currents