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Original question:

PGA305: Questions for PGA305

PGA305: ADC OUTPUT VALUES

shraddha k
Prodigy 180 points
Part Number: PGA305

Hello,

                  I am reading ADC values from PADC_DATA1/2/3. I use ADC in 24 bit mode. I have selected the digital interface mode. 

These are the values i use-

Input is = 4mV

Gain is 5 

Reference  = 2.5

So my expected value must be 0.004×5/2.5×7FFFFFh = 67109h. I don't get this value. I get random values . Also when i short the bridge , i don't see 0x00 as per section7.3.8.2 table 1

Also snippet of my code 

//compensation ctrl

IIC_RegWrite(0X40, 0x0C, 0X03);

//Bridge ctrl 2.5

IIC_RegWrite(0X42, 0x46, 0X01)

//Gain to 5

IIC_RegWrite(0X42, 0x47, 0X00)

//enable 24 bit mode

IIC_RegWrite(0X45, 0x88, 0X0D);  

IIC_RegWrite(0X45, 0x80, 0X01)
IIC_RegWrite(0X45, 0x81, 0X00) 
IIC_RegWrite(0X45, 0x82, 0X00)
IIC_RegWrite(0X45, 0x83, 0X00)
IIC_RegWrite(0X45, 0x84, 0X00)
IIC_RegWrite(0X45, 0x85, 0X00)
IIC_RegWrite(0X45, 0x86, 0X00)
IIC_RegWrite(0X45, 0x87, 0X00)

Can you please help out?

  • 0
    TI__Mastermind 31655 points

    Hello Shraddha,

    What are the values that you see when you read the data? And what order are you reading the registers? The data from the PADC must be read least significant byte first, so you will read PADC_DATA1 then PADC_DATA2 then PADC_DATA3.


    Regards,

  • 0 in reply to Scott Cummins
    Prodigy 180 points

    Hello,

                      Thanks for replying. I am reading in below sequence

    1.PADC_DATA1

    2.PADC_DATA2

    3.PADC_DATA3

    When i use in16 bit mode i read only  below register and form a value.

    1.PADC_DATA2

    2.PADC_DATA3

    so i shifted to 16 bit mode since i was not able to make any usable vaule out of 24 bit.

    with  16 bit ADC mode ,1 psi , brigde volatge 2.5 ,gain is 5v/v , no short

    ADC count = 225 or 0xE1

    when i short the bridge netowrk 

    ADC count = -39 or 0xFFD9

    I use the formula to calculate 

    0.004×5/2.5×7FFFh  = 263   The values(225 and 263) have lot of difference between them. Please guide me .

    Thanks

    shraddha

  • 0
    Prodigy 180 points

    Hello,

                      Thanks for replying. I am reading in below sequence

    1.PADC_DATA1

    2.PADC_DATA2

    3.PADC_DATA3

    When i use in16 bit mode i read only  below register and form a value.

    1.PADC_DATA2

    2.PADC_DATA3

    so i shifted to 16 bit mode since i was not able to make any usable vaule out of 24 bit.

    with  16 bit ADC mode ,1 psi , brigde volatge 2.5 ,gain is 5v/v , no short

    ADC count = 225 or 0xE1

    when i short the bridge netowrk 

    ADC count = -39 or 0xFFD9

    I use the formula to calculate 

    0.004×5/2.5×7FFFh  = 263   The values(225 and 263) have lot of difference between them. Please guide me .

    I think the offset is coming to play. How should be the offset be measured? how it should be used?

    Thanks

    shraddha

  • 0 in reply to Scott Cummins
    Prodigy 180 points

    Hello,

                    Is there anyone who can answer my questions? Please help out.

    Shraddha

  • 0 in reply to Scott Cummins
    Prodigy 180 points

    Hi,

                        Please some body answer my question?

    Thanks

  • 0 in reply to shraddha k
    TI__Mastermind 31655 points

    Hi Shraddha,

    There may be a little confusion in which register to read for the 16 bit mode. Your register read order is correct in 24 bit mode, but with 16 bit mode you still read only the two LSB registers (PADC_DATA1 and PADC_DATA2 in that order). Note that the sign bit will carry through to bits 24:16, so if the 16 bit output is negative, PADC_DATA3 will show 0xFF. This can be ignored. All of the 16 bit information is stored in the first 2 LSB registers.

    Please retry your measurements with the correct register reads for 16 bit mode, and let me know how your measurements turn out.

    Regards,