The QuadTech 1689 is probably unequaled for ac resistance measurements over its frequency range (12 Hz to 100 kHz). Unfortunately many precision resistance measurements call for dc instead of ac, even though ac measurements avoid thermal voltage errors, have lower noise and can use precise transformer‑ratio scaling techniques. Moreover, the size of the unit of resistance, the ohm, is determined from ac measurements and has to jump from ac to dc. Attempts have been made to use ac for the most precise resistance measurements, but the dc habit has been hard to break.

For most resistors, the ac‑dc difference is negligible at 100 Hz or even 1 kHz. For flat‑card wire‑wound resistors, the difference can be less than 1 ppm up to 1 Mohm if equivalent parallel resistance is used at high values to avoid errors due to lumped parallel capacitance and series resistance is used at low values to avoid errors due to series inductance. Lower measurement frequencies should also be used for very low values to avoid skin effect errors. There are significant differences for high‑value, coil‑wound resistors, because of capacitance not inductance, and for high‑value, multi‑resistor networks such as decade boxes and build‑up standards. The ac‑dc difference of resistance standards are generally very small and often can be easily determined by measuring it, and a small metal film resistor of similar value at both ac and dc. Here the assumption is made that the film unit has negligible ac‑dc difference (which it probably does) and that it was stable for the time required (which it usually will be if one doesn’t heat it up by applying too much power or touching it). Once such differences are determined, ac could be used for precision calibrations.