Hi Bastler
We typically recommend the following approach for reading RMS registers and comprehending them accurately:
1. When testing, change the inputs, wait for the readings to settle ( you need not read RMS values at this time; you can arbitrarily wait for ~500ms) and then read the RMS registers 100 times over a period of 1 second. Average the 100 readings and that will be your accurate result.
2. In application, when the current can change any time, keep collecting 100 values spread out over a second and update the LCD display (or however you report the RMS value to the end user) at the end of every 1 second time interval.
In your case, if accuracy is not a real concern, you can avoid the averaging part of the recommendation. Then all you have to do is to find out the max settling time taken for the readings to settle. Try out worst case conditions such as changing inputs from Max current to Min current and from Min current to Max current to see how much time it takes for the readings to settle. Let us say that it takes around ~500ms to settle. Then you can read the RMS registers once every 100ms (or at whatever rate you can afford to read) and update the LCD display.
With respect to regulating the transformer with a 'new' value, I am not sure if I understand your concern completely. The RMS register will get updated every 8kHz with a new value. Even after the readings are settled, there will be some ripple in the measurements; so every new value will be at least a little different from the previous value; that is why we recommend averaging over a second.
With regards to your second issue, it can either be a source issue or a software issue. From the IC's point of view, you cannot turn off one of the channels randomly. I am assuming that you are not using ADI's evaluation board for this experiment. So, can you share your board schematic and setup connection diagram.
If you feel that your source is accurate, let us start with the software. What is your code flow when you read these RMS values from all channels? Can you write a simple code to read one channel multiple times before moving over to the other channel? Try to alternate the order in which you read the channels and see if it affects the readings.
Regards
hmani