The 3400A RMS Voltmeter uses a thermal converter to achieve excellent RMS accuracy in AC measurement of inputs having crest factors up to 10:1. Voltage ranges are selected via a switch on the front panel. The front panel meter is calibrated in both RMS Volts and dB (600 ohms).

The instrument features a rear panel DC output which is proportional to the front panel meter reading, allowing it to function as an RMS AC to DC converter. This output is buffered, so external loading does not affect the reading on the front panel meter.

The 3400A makes use of a matched thermocouple pair to provide accurate RMS (heating power) conversion. A synchronous demodulator (chopper) amplifier is used in one of the gain stages. THe 3400A is unusual in that it combines vacuum tube, transistor, and integrated circuit technologies in the same instrument.