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ADS1675 input signal range

Other Parts Discussed in Thread: ADS1675

Hello,

I don't understand what is allowed input swing of thew analog signal for ADS1675?

Information's from datasheet:

Page 2: Analog I/O to AGND –0.3 to AVDD+0.3 (this means that input signal can range from cca 0 to 5V)
Page 3:Full-scale input voltage +-Vref
Page 4: Vref is typically 3V
Chapter: Data format states that input signal 
Vin=(AINP–AINN) can swing from +Vref to -Vref
Figure 54. Basic Differential Input Signal Interface states that differential input signal shoud
be superimposed over 2.5V offset voltage.
Now, if I understand this correctly this means that 2.5V offset is considered to be virtual input ground
or 0V. That means that maximum input in ADS is offset + Vref = 2.5 + 3 = 5.5V, and
minimum 2.5 - 3 = -0.5V. This means that I can never reach 7FFFFFh or 800000h
as digital output.
Am I interpreting this correctly or is there something that I have missed?
Best regards
  • The input to the ADC is a differential signal.  Typically, differential signals are biased at the common-mode voltage.  You have to be careful to not think of the differential voltage in terms of single-ended signals.

    So for this case, VCM = 2.5V and VREF = 3.0

    To achieve full-scale input (Vin = 3V):

    • IN+ = VCM + VFS/2 = 2.5V + 3.0V/2 = 4.0V
    • IN- = VCM - VFS/2 = 2.5V - 3.0V/2 = 1V

    This achieves VIN = IN+ - IN- = 3.0V

    To achieve VIN = -3.0V, you can simply switch the input voltage levels about

    • IN- = 4.0V
    • IN+ =  1V

    This achieves VIN = IN+ - IN- = -3.0V

  • Thank you Greg for your answer, this info really helps me :-)

    Things are much more clearer to me now.

    Considering page 2 of the datasheet I am assuming that is safe to apply

    full differential voltage swing from 0 to 5V, values from 4V to 5V and from

    0V to 1V will be clipped but ADS1675 input pins won't be damaged?

    best regards

  • The datasheet Absoute Maximum Ratings table (which is what you are referring to I believe) shows the operating conditions permissible for the analog inputs as (Analog I/O to GND)