Use of programmable gain amplifiers in ADE7878A

Hello 

I'm using ADE7878A. I have a question regarding the use of programmable gain amplifiers integrated in that IC. 

Suppose i develop a 3 - Phase Energy meter using that IC (ADE7878A)  and i connect 3 different load L1, L2 and  L3 to respective ADCs of the IC. All load has different current at a time such that current in L1 and L2 are approximately same but current in the L3 is so less that it can only be detectable at when PGA = 4. 

But datasheet of that IC says that we can't change tha PGA of Individual Current channels. so how the above condition can be implemented using ADE7878A ?

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  • Hi Dg,

    How small is current in Phase C? Will you be able to share your info, like dynamic range, how small from the full scale? is it a noticeable load?

  • hello 

    As per data sheet of ADE7878A, over a dynamic range of 1000 to 1 @ PGA = 1, error in RMS calculation of current is not more than 0.1%

    My Meter Max. current is 88.38 A, that means, in current range of 88.38A to 88.38mA, error % will be 0.1%

    Now suppose i want my Electric Meter to show Amps of as low as 0.020 A (say in Phase A). Now i want PGA of current channel to use. but at same time suppose current in another phase is 70 A (say in Phase B). So if i use PGA of current channel than ADC of Phase B will get a differential much voltage higher than 0.5 V (as indicated in datasheet this is highest voltage that can be applicable to ADC input pins) 

    this type of problem can occur where a 3 phase meter is connect to 3 different single phase load.

    So how can i implement such a problem?

  • Hello DG,

    what you can do is to design your phase at half dynamic ranges, then while measuring i think you can do switch PGA gain for the small currents, but take note of the change, at it should reflect on the real values conversion as well, that you did a PGA change, your microcontroller should be able to manage that. I have another customer experience where they use two cycles to detect the peak then they change the PGA, that 2 cycles at small peak should not have any large problem in the accumulation since the currents are small.

Reply
  • Hello DG,

    what you can do is to design your phase at half dynamic ranges, then while measuring i think you can do switch PGA gain for the small currents, but take note of the change, at it should reflect on the real values conversion as well, that you did a PGA change, your microcontroller should be able to manage that. I have another customer experience where they use two cycles to detect the peak then they change the PGA, that 2 cycles at small peak should not have any large problem in the accumulation since the currents are small.

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