The 74LS503 and 74LS502 ICs are successive-approximation registers, which can perform 8-bit analog-to-digital conversion using a digital-to-analog converter (such as the AD557) and a comparator. The 74LS503 has extra functionality, in that multiple chips can be “stacked” to perform conversion with higher bit resolutions. While this requires more complexity than modern “all in one” ADC ICs, it is useful as an educational setting.
Unfortunately, these ICs are no longer available. This page describes the emulation of these ICs using CPLD devices.