ADE7880 ,relation between the minimum current and the resistance ,voltage accuracy

 The experts:

   Ade7880 is not calibrated. 

    1. Input 230V voltage to ADE7880, Input 1A Current to ADE7880;  I got the data from the ADE7880,the Voltage is 234V,the Current is 1A, Please tell me ,is this currect ? thanks.

    2. current channel(IAP,IBP,iCP) of ADE7880,connected to current transformer(1:3000);when the resistance of IAP,IAN selected 7.5R*2,ade7880 can identify the input current of 90ma;

    however!!  when the resistance of IAN,IAP selected 2.2R*2,ade7880 can not identify the input current of 90MA, please tell me why?  i wound like to  know that the relationship between the minimum input  current and resistance.

    Thank you for your help

  

  • 0
    •  Analog Employees 
    on Dec 10, 2019 2:42 PM 11 months ago

    Hi Ehan

    If you apply 1A to a 3000:1 CT with a 7.5x2 burden resistor the output will be... 

    1A/3000 * 15 total burden = 5mvrms at the adc inputs.    Effective fullscale of the ADC   is  .3535rms/.005rms  * 1A = 70.7 amps 

    If you apply 1A to a 3000:1 CT with a 2.2x2 burden resistor the output will be... 

    1A/3000 * 4.4 total burden = 1.47mvrms at the adc inputs. Effective fullscale of the ADC   is  .3535rms/.00147rms  * 1A = 240.47 amps 

    I think the first thing is what is the maximum current you want to measure in your system? 

    max current *1.2 for some headroom in current measurement. 

    .3535 fullscale ADC / (max current you want to measure *1.2 for headroom / 3000 ct turns ratio) = total burden resistor

    With current channel  fullscale adc inputs of .3535vrms the IRMS codes are are 3,766,572 (0x39792C)

    (Imax current you want to measure *1.2) / 3,766,572=  IRMS per code.

    so if 30A is your IMAX 

    (30A *1.2 ) / 3,766,572= 9.557762e-6 IRMS/LSB  

    Now in theory 3,766,572 is the expected code at Imax, to calibrate you would adjust Igain to give you the expected IRMS code at a given calibration current.

    If your calibration current is 3 amps 10% Imax for this example. the expected IRMS code are 376,657 from the IRMS register. Adjust Igain to give you 376,657 at 3 amps RMS. 

    IRMS performance is .1% at 1000:1  in this case 36/1000 = 36maRMS should still be within .1%. You would also have to do IRMS offset correction  if needed to adjust for any noise or crosstalk see pg 43 

     

    Hope this helps. 

    Dave

  • Hi Dave,Thanks for  your help.

     i have one product, the product supports three current specifications, by replacing current transformers. the current transformers specifications is 60A,100A,200A; the CT of 60A  can identify min current input of 30MA, the CT of 100A can identify min current input of 50MA,the CT of 200A can identify min current input of 90MA;I'm very troubled by the choice of the load resistor ;

     in your way,I used 200a as a benchmark,so the burden is: .3535/0.08 = 4.4187R; IRMS per code = 240A/3766572=0.0637MA,  So...I think: Only one burden is used  for three current transformers.

    if the burden is 4.4R,the ade7880 can also identify the input current of 30 ma?

    Or can you give me a better idea?

  • 0
    •  Analog Employees 
    on Dec 11, 2019 4:20 PM 11 months ago in reply to Ehan彭

    If all your cts have the same turns ration of 3000 to 1 for 200 amps 100 amps and 60 amps and you use the same burden 4.4ohms. 30ma is 8000:1 from full scale of 240Amps. The ADE7880 IRMS is specified to .1% at 1000:1. With a full scale of 240A FS and   1000:1 current would be 240ma,  At 30ma (8000:1) you will get something but not within .1% or .5% 

    At 30ma you will get 470 codes from the IRMS register. 1lsb is .2% error at 30ma

    What is your accuracy requirement for the 30ma,50MA and 90ma measurement? 

    Dave