| Voltage comparator circuits are common integrated circuit (IC) building blocks found in ICs used in a variety of applications, including test and measurement instruments, wireline communication systems, and data converters. High-performance comparators are often fabricated in high-bandwidth bipolar processes, which are typically very susceptible to the effects of self-heating. Self-heating in comparator circuits manifests itself as signal-dependent propagation delay variation, which appears at the comparator output as data-dependent jitter. For comparators used in applications where precise timing measurements of threshold crossings are sought, self-heating is an issue that must be addressed.;A circuit-based self-heating compensation scheme applicable to asynchronous comparator circuits has been designed, simulated, and implemented on a test chip fabricated in IBM's BiCMOS8HP SiGe HBT IC process. This compensation scheme differs from prior work addressing self-heating-induced errors in comparator circuits, in that it is applicable to asynchronous, i.e., non-clocked, comparator circuits. It also represents an improvement over simpler compensation schemes commonly applied to non-clocked comparators, in that it is insensitive to input signal swing and common-mode variation. The central element of the self-heating compensation circuitry is a power-to-voltage converter (PVC) circuit that enables the generation of a feedback signal to provide self-heating compensation.;Initial measurements of the test chip indicated that the comparator was over-compensated for the effects of self-heating. It is suspected that the excess compensation is due to differential thermal resistance mismatch between the amplifier transistors being compensated and those in the compensation circuitry. It is believed that the thermal resistance mismatch is due to the effects of different metal-layer interconnects for the two pairs of transistors. A work-around was identified to reduce compensation path gain below its minimum designed-for value, allowing the self-heating compensation circuitry to be calibrated, even in the presence of the unexpected thermal resistance mismatch. Measurements of the comparator, both with the compensation circuitry enabled and with it disabled, showed that the self-heating compensation circuitry presented here provides effective compensation of self-heating-induced timing errors over a wide range of input signal conditions. |
