I am a newbie to ADE7758. I have got some problems:
1. I do not understand how to use Calibration for calculating conversion constant for RMS value in xVRMS and xIRMS to get the real - world value.
2. In ADE7758 datasheet, section ROOT MEAN SQUARE MEASUREMENT, am I right that I determine the value that represents the full - scale value for VRMS register at 50Hz is 0x193504 = 1 651 972d? I see other value at Table 9, but I do not understand what it really is.
3. Also in section ROOT MEAN SQUARE MEASUREMENT, I can not see the equivalent full - scale RMS current at 50Hz.I truly appreciate someone explain these to me.Thanks in advance!
You three questions pertains to on how to convert the codes into real world values basically the conversion constant like Amperes / code (Amperes per code) or Volts/code
THe maximum Analog differential sine wave signal entering the ADE7758 should be 0.5 Volts (+ or - )
meaning if the Singal enters the channel as 0.5V peak, you will get full scale codes, as an example, on page 29, the o.5Vpeak on the voltage channel produces 1639101 codes in the VRMS register
Then in your design for example a voltage channel, 240V with 1000:1 Potential Divider ratio, the value that will enter to your ADC is 0.24Vrms or 0.339Vpeak, if you do ratio and proportion, your 240Volts corresponds to 0.339V peak that enter the chip, then you are expecting the if 0.5Vpeak enters the chip, the real voltage value prior potential divider is 353.98Volts.
To create a conversion constant when you are 0.5Vpeak you expect that your will have a full scale Vrms codes which is 4191910 (if in 50Hz for example). This means that 353.98Volts is 1639101 codes, your cnversion constant is 353.98 Volts/1639101 codes or 215.96 uV/code. Meaning each 1 bit or LSB code corresponds to a 215.96 uV.
Thanks for replying me.
But I still have no idea about the full scale code stored in xIRMS at 50Hz. Could you give me the information about it?
And one more thing, is it right if I do the following steps to get the real - world value RMS:
Step 1: Get the offset value stored in VRMSOS / IRMSOS in calibration stage initially.
Step 2: Store the conversion constant in MCU.
Step 3: Read the RMS value then add offset value following equation (9) or (10) to get the code - value, putting back to RMS register.
Step 4: Get the real world value by mutiplying the conversion ratio and RMS register value.