Post Go back to editing

Power vs Energy calibration

Category: Datasheet/Specs
Product Number: ADE7880

Hi all,

In the ADE7880 datasheet the calibration registers contain xPGAIN (Phase x power gain adjust) registers.
These registers are indeed used in both Power and Energy calculations.

However, in AN-1171 (Calibrating a Three-Phase Energy Meter Based on the ADE7880) these registers are consequently used for Energy gain calibration.
And for the reasoning of doing the calibration: the purpose of the energy gain calibration is to compensate for ... e.g. time error introduced by the crystal, which indeed indicates Energy rather then Power.
Also the procedure involves energy measurements rather then power measurements.

A few questions:

  • Why are the Power calibration register values registers calibrated as (time dependent) Energy?
    Wouldn't it introduce errors with the Power calculations (as Power is time independent)?
  • Is there a reason why the timebase itself isn't calibrated?
    With doing that also other time-dependent measurements (e.g. period) would be calibrated.
    If timebase calibration is needed anyway, as a crystal seems to be accurate enough by itself.
  • Wouldn't it be better to use Power measurements to calculate the xPGAIN register values (and assume the crystal accuracy to be good enough)?

Thanks in advance.

  • I typically calibrate Igain and Vgain based on the sensors/voltage dividers transfer function and expected RMS codes. If this is done correctly you will not need power gain, you may need power offset at low currents. I also suggest you do phase calibration first. The phase calculation is using ratios, so It does not require gain calibrations first. and don't use teh 10^n in the calculation. you want the most codes possible for best accuracy. I suggest accumulation of 0.2 to 0.5sec usign linecycle accumulation in the ADE7880, then read values and accumulation in the micro. 

    The crystal is good enough and as I have stated if igain and vgain are done correctly you should not need pgain. Pgain is basically factors to IGain * VGain

    Dave

     

  • Hi Dave,

    thanks for your reply.
    I indeed also would expect that after phase and voltage/current compensation, power calibration is not needed really, as power only depends on those units, and I do not see any other source of error in the power calculation.

    But as the power gain registers exist, I would like to know why they exist, so I can assess the risk of not using them (they exist for a reason I suppose).

    As we only work with/are interested in higher currents, offsets are not relevant for us.

  • pgain is there as a legacy feature. Some older applications may not need IRMS and VRMs or power. so pgain would be used to calibrate if only active energy and cf pulse is needed. Igain and VGain perform the same function as pgain if calibrated correctly.

    Dave

  • Aha, that makes sense.

    Would be nice to have that information added in the datasheet (or AN-1171), it saves a lot of calibration time during production.

  • www.analog.com/.../AN-1171.pdf

    There is a section in the rms calibration 

    You will find if you follow this advice, when verifying the power after Igain and vgain calibration, no pgain will be needed. 

    Dave

  • That part actually made me wonder whether PGAIN calibration is still needed, as it says any adjustments to xIGAIN and xVGAIN register should be done prior to calibrating the energy, which indicates (to me) that energy calibration is still needed after xIGAIN and xVGAIN calibration...

  • The point I am trying to make is if you have calibrated Igain and VGain correctly. When you try to do pgain calibration you will find the error is extremely small, so it does not need to be done. 

    There for it is not needed. You can check this yourself.

    Dave

  • Hi Dave,

    I totally get your point, thanks for that.

    My point is that (at least to me) it was not clear from the documentation that calibration of PGAIN is not needed when VGAIN and IGAIN are calibrated.
    For sure we would eventually find out that calibration of PGAIN would never be done as the error would always be very small, but the only way for us to find out is by verifying it in production, which would take valuable time.

    Now that we know PGAIN calibration is not needed at all, we can skip the verification/calibration, and save a lot of time.