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ADS1220: ADS1220- Input voltage - Read value

Roshan Anand
Prodigy 30 points
Part Number: ADS1220

Hello

I am configuring the ADC according to the following  to read the input voltage provided at AIN0:

Config Reg 0: 0x81

Config Reg 1: 0x04

Config Reg 2: 0xD8

Config Reg 3: 0x00

I am using the ADC in unipolar mode: AVDD:5V; AVSS:0V; DVDD:3.3V; DGND:0V

I am getting following values for the input voltage provided @AIN0:

0V: 0x7FFFFF

5V: 0xBFFFFF

But, according to my understanding for an input voltage of 5V, respective read value should be 0xFFFFFF right?

Is my understanding correct or am I missing out something?

Thank you in advance.

Regards

Roshan Anand

  • 0
    TI__Guru 55342 points

    Hi Roshan,

    Please allow us some time to respond as many of our engineers are out for the holidays. Thank you. 

  • 0
    TI__Guru** 113765 points

    Hi Roshan Anand,

    The ADS1220 always measures the input differentially regardless of if the input is single-ended.  That is to say the output code is based on AINP relative to AINN.  The conversion result is binary 2's complement where positive codes range from 0 to 0x7FFFFF (AINP > AINN).  Negative codes range from 0xFFFFFF (-1) to negative full-scale 0x800000.

    With the reference voltage at 5V, the full-scale range is 10V (+/-5V) and when connected as single-ended AINP can never go below AVSS so only the positive codes (1/2 of the full-scale range) will be provided which is 0 to 0x7FFFFF.  It is possible that with a 0V input there can be some negative code values if there is some negative offset for the ADC.

    So I would expect for a 5V input that the code returned would be positive full-scale (or very close to it) and with a 0V input very close to 0 (unless there is a negative offset and then you may see something like 0xFFFF18).

    If this is still unclear, send some oscilloscope or logic analyzer shots of the communication when reading the conversion results and share what the input voltage was when the shot was taken. Also verify that the communication correctly follows the SPI timing as shown in the datasheet where the data changes on the rising edge of SCLK and remains steady on the falling edge of SCLK.

    Best regards,

    Bob B