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The problem is that they give a step on the PT100 and the digital filter has a certain settling time. So each time they change the channel, they will have to wait for 3-4 samples to read out correct values, cycles depending if they synchronize a filter reset with the step given to the sensor Would you confirm this?
When a step change occurs on the AD7715, the filter will require 3 conversion cycles to settle. If DRDY is high before the channel change, DRDY will stay high for 3 conversion cycles after the channel change. When DRDY goes low, a valid conversion will be available. However, with the AD7715, if DRDY is low before the channel change (i.e. another conversion has been completed before the channel change occurs), the DRDY line stays low following the channel change - it is not reset high. In this case, a dummy read is required. The dummy read of the data register is needed before correct data gets loaded into the data register. So, in summary, one invalid reading may be received due to the dummy read. However, if sufficient settling time is allowed after the channel change, the second reading (after the dummy read) should be valid.