AD777x: How a decimal ODR is achieved if only an integer number of DCLK periods can be count?

If the decimation factor N is a non-integer number, the period of the DRDY oscillates between the periods determined by the integers obtained rounding up or down the decimation factor.

 The average of the periods is equal to the expected ODR. The periods, however, are always calculated at the ODR programmed to the AD777x.

 For example, in low power mode if N = 160, ODR = fMOD/N = 512/160 = 3.2 kHz and the period between DRDY pulses is 1/3200 = 312.5 μs. If N = 159, ODR = 512/159 = 3.220126 kHz and the period between DRDY pulses is 1/3220.126 = 310.547 μs.

 If now we select a non-integer value of N = 159.968006, the DRDY period will automatically oscillate between the period determined by N = 159 (310.547 μs) and N = 160 (312.5 μs) in such a way to average N/fMOD = 159.968006/512000 = 312.438 μs. The data output is already internally interpolated to match the expected ODR, so external postprocessing is not required.

It is possible to estimate the number of DRDY pulses required to see a jump in the ORD. It depends on IF (fractional number of N).

When IF<=0.5
When IF is>0.5
Using the values from the previous example:
IF=0.968006 --> Pulses=1/(1-0.968006)+1=32.35. 
So every 33DRDY pulses a ODR jump from 160 to 159 should be seen. 

For more information refer to:AN-1388 (Rev. 0) (analog.com)

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