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ADS8528 Acquisition time in serial mode

Jigar Patel1
Prodigy 130 points
Other Parts Discussed in Thread: ADS8528

I am trying to find total conversion time with ADS8528. I am referring "SBAS543A–AUGUST 2011–REVISED OCTOBER 2011" datasheet for reference. 

Page number 10 : Total acquisition time is 280 nSec. (With Serial mode)

Max Clock frequency is 15 MHZ or 66.67 nSec

To get 12 bit data at 15 MHz it takes at-least 12 * 66.7 nSec =  800 nSec

Can somebody explain how 280 nSec acquestion time is possible in serial mode.

-Jigar

  • 0
    TI__Guru**** 181831 points

    Hi Jigar,

    The 280ns acquisition time noted on page 10 is the minimum acquisition time needed to ensure you properly sample the inputs, and this covers all three devices in the family.  The 15MHz clock you noted is the maximum frequency of the external conversion clock, the actual serial clock used to clock out the conversion results can be as high as 45MHz.  If you are using all four serial outputs, you actually want to clock out both CH0 and CH1 from SDOx, and that would be 32 clocks - the 12 MSBs followed by zeros in the case of the ADS8528.

    So, the conversion time is based on the MAX 15MHz clock and it takes 19/20 cycles, which is where the 1.33us maximum conversion time comes from.  For the ADS8528, the 480kSPS maximum data rate is a cycle time of 2.08us, which actually leaves you with 2.08us - 1.33us = 750ns sampling time (the device goes back into sample mode while you are clocking out data).