There are six major blocks in the device: The range selector, RMS-to-DC converter, ADC, microcontroller unit (MCU), keypad and LCD display. The range selector scales the input voltage down to a known value, under control of the MCU. It also includes protection circuitry, to prevent damage to the rest of the circuit in the event of a high voltage being applied with a low scale-factor selected. The scaled voltage is then fed to an AC/DC selection switch. This allows the MCU to choose either DC or true-RMS AC measurement. The selector's output is fed to the ADC's input. The ADC used is a MAX132 18-bit plus sign ADC with serial I/O. The ADC's output is read by an Atmel 90S8535 MCU, which does the required data formatting, conversion and storage functions. In addition, it implements the user interface via a 16x2 alphanumeric LCD, telephone keypad and an RS-232 link.