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ADS1231: Understanding ADS1231 Output

Pablo Fuentes
Prodigy 90 points
Part Number: ADS1231

Hello! I'm trying to get some outputs, but I think I don't understand the values.

I'm trying to give a 0 V input so the ADS can give me the value of 000000h. The schematic is the following:

Where AINP-AINN = 0 mV, so the output should be 0. The SCLK signal I'm sending when I detect low in DRDY is a 2ms Period signal, so data can be read without being overwritten.

This value I'm getting is the following:

 

where blue signal is DOUT and yellow one is SCLK.

Is there anything I'm setting in the wrong way?

  • 0
    TI__Guru** 113765 points

    Hi Pablo,

    I cannot get enough detail in the scope shot to tell what the horizontal time base is that is being used.  Can you tell me what the unit of time per division that the scope is set at?  Also, it is not clear from the schematic what the SPEED pin setting is that you are using.  If you are indeed using a 2ms period for SCLK, then the SPEED must be set to low (10sps data rate).

    For a 0V input you must also make sure that the input doesn't float.  In other words, you cannot just tie AINP and AINN together.  Also, you cannot connect the inputs to ground as this will be outside of the input range for the ADS1231.  The input voltage must be within AVDD-1.5V and AGND+1.5V.  The best way to check the shorted input is to create a voltage divider using 10k Ohm resistors where the junction of the two resistors is mid-supply.  At that point you can tie both the AINP and AINN inputs together so that the inputs are effectively shorted together for a 0V input.

    As to the actual output code, the code is in binary 2's complement.  As the input is differential the output code can be either positive or negative.  If there is a negative offset then the code result could be negative for a shorted input.  The positive range is from 0x000000 to 0x7FFFFF and the negative range is 0xFFFFFF to 0x800000 (negative full-scale).  There is only a 1 code difference between 0x000000 (0d) and 0xFFFFFF (-1d).

    Best regards,

    Bob B