# For which frequency is ESR specified by default?

I'm playing around with an REF5010 voltage reference, and its data sheet specifies an output capacitor with an ESR of 1 to 1.5Ω for best performance. Now when I measure the ESR of electrolytic capacitors it varies widely depending on the frequency (I have an U1733C LCR-meter that can measure at 100Hz, 1kHz, 10kHz and 100kHz).

The Wikipeadia article on ESR says

If not otherwise specified, the ESR is always an AC resistance measured with standardized frequencies.

but fails to provide details.

Which frequency (maybe depending on circumstances?) is used for determining the ESR of capacitors?

• I'm contemplating using the REF50xx device and I'm considering using a ceramic 10uF. The spec says below 1.5$\Omega$ but best noise is between 1$\Omega$ and 1.5$\Omega$ so I'm thinking of having an 0603 1R resistor in series with it. Oct 1, 2013 at 12:20
• @Andy aka The REF50xx specification says that 1µF is enough. Since this gives a relatively stable ESR over a wide range of frequencies it is probably a good idea, though with a ceramic capacitor I'd be worried about acoustic noise. Oct 1, 2013 at 14:32

However, general purpose capacitors seldomly have ESR as a statistic in their datasheets. They often only have the loss tangent (tan $\delta$) in the datasheet. This is measured most often at 120Hz, but in some cases also at 1kHz. There is usually no use in trying to measure these capacitors at 10 or 100kHz, because the internal construction prioritizes volumetric capacity over impedance, and the ESL makes the capacitors virtually useless at higher frequencies.