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Original question:

ADS1220: How to vary vref on upto 5v for external reference

ADS1220: How to use 5v for external reference and read 0 to 5v

HUMBERTO SALAZAR
Prodigy 10 points
Part Number: ADS1220
Other Parts Discussed in Thread: REF3450, ADS1118

Hello i need help for using the vref and read 0 to 5 volts with ads 1220 
i am trying to search and find the way to do it but i cant get any clue 

im using the arduino nano and protocentral library and i dont know whta do i have to change to enable VREF  and read 0 to 5 volts 
any suggestion ? Tutorial ? 
I will apreciate all the help 

#include "Protocentral_ADS1220.h"
#include <SPI.h>

#define PGA         1                 // Programmable Gain = 1
#define VREF        2.048         // Internal reference of 2.048V
#define VFSR         VREF/PGA
#define FULL_SCALE   (((long int)1<<23)-1)
thanks and regards 
  • 0
    TI__Mastermind 24468 points

    Humberto,

    The input range of ADS1220 is -Vref/Gain to Vref/Gain.  If you use the internal 2.048V reference and Gain = 1, the range is -2.048V to +2.048V.  Note, the range is the differential input range.  This means VIN = V(AINP) - V(AINN).  So, for example, if V(AINP) =1V and V(AINN) = 2V, than VIN = 1V - 2V = -1V.  The absolute input range on any one input is approximately AVSS to AVDD (0V to 5V if AVDD=5V, and AVSS=0V).  All signals need to stay in the absolute range for operation.  If  V(AINP) =4V and V(AINN) = 3V, the two inputs are inside the absolute range (between 0V and 5V), and the differential signals is VIN = 4V - 3V = 1V.  For this example both the absolute and differential range are ok for a 2.048V reference.   On the other hand, if you applied V(AINP) =4V and V(AINN) = 1V, this would be ok for the absolute range but not for the differential range VIN = 4V - 1V = 3V (doesn't work on a 2.048V reference.  Finally, if you applied V(AINP) =1V and V(AINN) = -1V, this would violate the absolute range (not between 0V and 5V), but will not violate the differential range (VIN = 1V - (-1V) = 2V).  You need to adhere to both the absolute and differential input range for operation.

    If you want to read 0V to 5V using single ended operation, you will need to use an external 5V voltage reference (cannot use the internal reference for this case). Single ended operation means that only one input is used and the measurement is taken relative to ground.  REF5050 is a precise voltage, and REF3450 is a lower precision lower cost reference.  These could be used for ADS1220 to get a 0V to 5V range.

    One other simple option is to use a voltage divider on the input to reduce the input signal level to less than 2.048V,  The main problem with this is that the voltage divider will have error due to the tolerance of the resistors.  If a voltage divider is used you can then multiple all your results by a factor to account for the divider.  

    If you are flexible on your ADC you could consider the ADS1118.  This device allows for voltage up to 6.114V on a single 2V to 5.5V supply using only the internal reference.  This device is only 16 bits where as the ADS1220 is 24 bits.  I don't know what your goal is, but in many cases the accuracy and capability of this device may be sufficient.  Of course there are many options, but I just mention this as one very simple alternative.

    Best regards,  Art