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Issue on IRMS using ADE7953 when the value exceeded the full range

Question asked by karens on Apr 7, 2014
Latest reply on Apr 8, 2014 by hmani

Hello,

 

On my project, I use ADE7953 to read IRMS and VRMS value. Upon the data sheet, "with full-scale inputs on current channel A and B, the expected reading on the IRMA and IRMSB register is 9032007d ...". During the initial design, the full current range is up to 34 amps. So I use 34 amps as the full current range when I calculated the current amps upon IRMS register reading. I set the gain to 2.

 

current value (without calibration) = (34.0 / 9032007) * IRMS-register-reading;

 

It works very well.

 

Now, with the new application, the full current range could go to 60 amps, or even higher. I noticed that after the current exceeded 34 amps, then reaching the certain level, e.g. above 50 amps, then the reading would go down, e.g. 27 amps although the load should be higher than 50 amps, I am not sure why the current value was going down after reaching the 34 amps current range since 34 amps as the full current range was set by my software. My questions are:

 

1. Was it caused by the fact that the IRMS reading was exceeded 9032007d (exceeded the full scale input)?  I thought at this case IRMS register reading could keep going high, when reaching 9032007d, it will stay at this value. But It looks like it was not. Will it be true that the IRMS register reading could exceeds 9032007d since the full range value of 24bit register value is 16777215d(0xFFFFFF)? 

 

2. I think there are two ways to solve this issue. The first one would be to change the current range setting from 34 amps to higher value, e.g. 60 amps. But it will sacrifice the resolution on the small current measurements. Another way is to change the gain setting (PGA and PGB) to 1. Are these methods will solve this issue, or is there any other better solution?

 

3. If I could solve the issue by changing the gain setting. How long should I wait after the change (need settling time)?

 

Thank you very much for the help!

 

Karen Shi

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