What is 'minimum RMS current' in STPM32 datasheet?

[DGiov.1]

Hello,

I'm working with the STPM32 and I don't understand what I MIN (also referred as resolution) is (Equation 27, page 79 of DS10272 Rev 8).

Why if the data is provided in the registers with 17bit resolution, the calculated RMS current resolution is two bit less?

In my case I MIN is 2.058mA, and on the device I'm working on I measure 2-3mA even if no load is applied. Is the 'no load' rms current supposed to be always above I MIN?

Davide.

[Marilena Gaetana SAMBATARO]

Hello,

the LSB of the RMS current register is calculated considering 17 bits (see datasheet pag. 52). Normally this value is not measurable because of the AFE noise and the internal RMS block resolution, so the minimum RMS current that the device is able to measure is indicated as Imin and it is bigger than the LSB, most likely 4 times.

Please consider that the energy resolution is much bigger, since it is calculated from 28 bits ADC data.

Hope this helps you.

[DGiov.1]

Thank you,

yes it is more clear.

But what about the open circuit current? With no load (open circuit) and with a zero mean noise, I expect to read 0mA, but I always measure at least 2-3mA. Is this the effect of the zero mean noise injected into the RMS block?

[Marilena Gaetana SAMBATARO · Accepted solution]

Yes, correct. The zero mean noise produces a positive RMS value.

[DGiov.1]

Than you very much.

Davide.