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RMS to DC Converter

John Griffith
Mastermind 21555 points
Other Parts Discussed in Thread: ADS7028, ADS7128, LM13700

Hello,

My customer is looking to implement an RMS to DC converter to monitor the output power applied to an external load. I looked around on E2E and for app notes on TI.com and haven't found a lot:

E2E post (we have one PADC with RMS calculation but has more channels than needed): https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/910110/rms-to-dc-convert-solution 

App notes (some outdated app notes from decades ago):

https://www.ti.com/lit/an/snoa690/snoa690.pdf

https://www.ti.com/lit/an/snoa592c/snoa592c.pdf

Has anyone done a more recent simulation or design of implementing this with more modern op amps? If I understand correctly the top app note is rectifying the voltage and then has a a stage to multiply / divide. I was thinking we might have some newer solutions we could propose. 

John

  • +1
    TI__Guru 62915 points

    Hi John,

    There are non-TI devices that perform direct RMS-to-DC functions.  At the time of this posting, TI does not offer a device with an analog RMS output.  However, as you noted the ADS7128 and ADS7028 devices include the RMS calculation internally and can output a digital output directly.  The RMS-to-DC devices all need to get sampled by an ADC anyways so combining the RMS engine into the ADC itself combines components.  The pricing of these devices is also on-par, or better, than many of the RMS-to-DC functions in a smaller package. 

    We'd like to see more traction with this solution and information on why it's not a good fit would be appreciated.

    https://www.ti.com/lit/ds/symlink/ads7128.pdf

    See section 8.3.12 Root-Mean-Square Module for more information:

    • Root-mean-square module:

    -16-bit true RMS output

    -Programmable RMS time window

  • +1
    Guru 140200 points

    Hi John,

    the LM13700 can be used to build a simple RMS converter:

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 21555 points

    Hi Collin and Kai,

    Thanks for the quick responses. I have proposed both to the engineer I am working with. Collin, I will let you know via email if I get some feedback on this solution. My "fear" is that they are only monitoring one signal and don't need the 8:1 MUX this device has integrated but we shall see!

    John