AD73311L: Cascade running at different sampling speeds

Document created by analog-archivist Employee on Feb 23, 2016
Version 1Show Document
  • View in full screen mode

We want to use two 73311L's in cascade operation but with different sampling
rates.(to be specific 8 kHz and 48 kHz) Could you please inform us whether this
operation possible?


The short answer is No, here’s why.

Each device in the cascade must know how many devices are in the total cascade,
in this case 2. In each sample interval each device will accept a total number
of data words equal to the number of devices in cascade. When a device receives
the last word (i.e. the number of words equal to the cascade length) it ignores
all other words.

In normal operation both devices in the cascade have a sample ready at the same
time. The first device in the cascade passes it ADC word to the second device,
and the second device passes it ADC word to the DSP. At this time Device 1 has
received no words and Device 1 has received 1 word. Because the total length of
the cascade is 2, Device 2 knows that it should receive more than 1 word, so it
passes this word (the ADC word from Device 1) to the DSP. The DSP may also
write DAC words to the cascade. The first DAC word is written to Device 1. At
this time Device 1 has received 1 word, but like Device 2 it knows that the
total cascade length is 2, so it passes this word to Device 2. Now Device 2 has
received 2 words and keeps this word as a DAC word. The DSP then transmits a
second word to Device 1. Device 1 has now received 2 words, so it keeps this
word as a DAC word. If the DSP sends any more words to Device 1, they will be
ignored because Device 1 has received all the words it wants in that sample

In the suggestion above, if Device 1 is running at a higher sample rate than
Device 2, in one sample period of Device 2, Device 1 will transmit 2 ADC words
to Device 2. Device 2 will think that the second ADC word from Device 1 is
actually it's DAC word, and will not pass this word on to the DSP. Similarly,
if it is Device 2 that is running faster, the DSP will not be able to write
multiple ADC words to Device 2 in one sample interval of Device 1.

Also, you can't achieve 8k and 48k sampling from the same DMCLK. All devices in
cascade have to have the same DMCLK. Sample rate options are DMCLK/2048,
DMCLK/1024, DMCLK/512 and DMCLK/256.