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ADS1244 full scale

Wilson Chiu39
Prodigy 220 points
Other Parts Discussed in Thread: ADS1244

Hello, 

I m using ADS1244 for voltage reading. 

If i use

AVDD=5V

DVDD=3.3V

Ref (+)positive = 2.5V 

Ref (-)negative= 0V. 

AINN=0V

 

Question 1 ) AINP range should be +/- 5V.   Is this correct?

 or AINP is limited by AVDD/2 ?     (Datasheet states   +/- 2 Vref)

Question 2) according to the setting, it should output   7F FF FF for  AINP=5V?   

and    FF FF FF for AINP = -5V    ????

Question 3) This setting for ADS1244 is capable of measuring from -5V to +5V ? 

  • 0
    TI__Guru* 93735 points
    Wilson,


    1. The ADC range is +/-5V but may not mean what you think it means. The ADC is fully differential so that if AINN=-5V, and AINP is 0V, this would measure -5V. The absolute maximum ratings of the part (shown on page 2 of the datasheet) say that the lowest any analog input can go is -0.5V and the highest the analog input can go is AVDD+0.5V. Under no circumstances can the analog input voltage go to -5V. Since the absolute maximum may push a part to the point of destruction, you should note that the range of operation is GND-0.1V to AVDD+0.1V

    2. At the positive full scale, AINP=5V, AINN=0V, the ADC would read 7FFFFFh, while at the negative full scale, AINP=0V, AINN=5V, the ADC would read 800000h. Note that with these values the input common-mode voltage is +2.5V. The data comes out in a two's complement notation. You can see a table of input voltage compared to ideal output codes on page 10.

    3. If you have any analog input that is at -5V, then no, you cannot measure that. You will need build a buffer stage that will lower the gain to the input and level shift the voltage to something where both AINN and AINP are within 0V to 5V.


    Joseph Wu