AD7470 Acquisition time


I am confused about the acquisition time specification for the AD7470 - it's given to be 135ns. My understanding of ADC is that the acquisition time is related to the settling to 1LSB of the sampling circuit.  The circuit given in the data sheet for the sampling circuit has a time constant of a few [ns].   So for 12-bits converter will require appox. 8 time constants for 1LSB settling which would be of the order 40[ns]. Can someone explain where the 135[ns]  is from? Thank you.


  • 0
    •  Analog Employees 
    on Oct 23, 2018 7:16 AM

    Hi Mike,

          Apologies about your confusion. In the ADC SAR data processing, there is a sampling stage, this is where an analog voltage is sampled by the ADC and this is also the voltage to be converted. There is also the acquisition phase, this is the time the ADC will be acquiring a voltage level that needs to be converted. When the ADC wanted to convert an input voltage , it would want that the input voltage have settled to its desired voltage before acquisition expires, which in this case 135ns. You are right, for a 12 bit it requires that the voltage should have settled to 1LSB at 8 time constant. The 135ns mentioned in the Data Acquisition time section of the datasheet was characterized value, considering the components shown in the figure and including also other design factors  in the AD7470.