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ADS1015 / relation of Vin and VDD

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: ADS1015
Hello,
 
I'm Ryuji Asaka.
Could you please kindly let me know the 1LSB value of below example ?
 
-----------------------
VDD=3.3V
VIN=3.3V ( single-end)
PGA SETTING =1 (It mean FSR=±4.096V)
 
1LSB=3.3V/4096(12bit) ?
                 or
             4.096V/4096 ?
-----------------------
 
Best Regards,
Ryuji Asaka
 
  • 0
    TI__Guru** 113765 points

    Hi Ryuji,

    In this case the full-scale input voltage range is +/- 4.096V (or 8.192V total) divided by the total bits available from the ADC.  8.192V/4096 = 2mV per LSB.  Keep in mind that you cannot apply more than 0.3V more than the supply voltage.  So even though your range is up to 4.096V you can only measure up to 3.6V maximum.  Also you lose one bit of dynamic range when measuring single ended.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Guru 20090 points

    Hello Bob san,

    Thank you for the reply.
     
    So, in this case the output code of 3.3V single end input is as below ?
    (3.3V+4.096V)/2mV  = 3698 ,  out put code is 3698th code of  total 4096code(12bit).
     
    3698th code = 111001110010    in binary code.
     
    Best Regards,
    Ryuji Asaka 
  • 0 in reply to Ryuji
    TI__Guru** 113765 points

    Asaka-san,

    The typical ADC transfer function is:

    Output Code = AIN/VREF*2^(n-1) where n is the number of bits in the system.  As the ADS1015 does not have a typical reference value, you would use the full-scale value. For your configuration the result is:

    Output Code = AIN/4.096V*2^(11) and for 3.3V the output code is 1650 (and 1650 times 2mV = 3.3V.)  1650 is 0x672 or b011001110010.  Keep in mind that the ADS1015 measures differentially, and a single ended measurement reduces the range to 1/2 of the total available codes.  On the positive side of the range the measurement is from 0 to 2047 codes.  If the MSB is high, then you have a negative value returned.

    Best regards,

    Bob B