AD7709 CONVERTER PROBLEM

Hi,

i use an ad7709 to measure thermocouple signal, and there is a problem:

conditions:

1. input channel:AIN1 and AIN2.

2. use pseudo-bipolar mode.

problems:

1. when i operated as follow process:

     hardware reset (use reset pin ) --> read first data --> read second data --> end

     then, both datas were wrong.

2. when i only read one data,

     hardware reset --> read first data --> end

     then, the data was right.

so why?

and i also have some questions:

1. do i need to calibrate the product after manufacture?

     i read datasheet that, the device is factory-calibrated at final test. however, we will use different voltage reference, is that still ok?

2. do i need to care about zero shift?,

     in most ad converters, this is a serious problem, so is it same with this device?

Thanks.

  • 0
    •  Analog Employees 
    on Nov 18, 2013 6:26 PM

    Hi,

    We are looking into this. We'll get back to you as soon as possible.

    Thanks and Best Regards,

    Chris

  • Hi,

    I have changed SPI communcation frequancy, and data is right now.

    but i didn't found any explanation about spi frequancy.

    However, i still confused about zero shift, so i still need your help.

    Thank you.

  • 0
    •  Analog Employees 
    on Nov 25, 2013 4:28 PM

    Hi,

    What data are you reading from each channel?

    When reading two channels the sequence should be:

    • Perform a reset
    • Write 0x22 to communications register: This indicates that the next operation is  a write to the filter register
    • Configure filter register: set required output data rate

    Read first channel:

    • Write 0x01 to communications register:  This indicates that the next operation is a write to the configuration register
    • Set channel selection bits(CH2:CH0) in the configurations register to 0b000 to select AIN1 - AINCOM
    • Poll /RDY pin
    • Write 0x43 to communications register:  This indicates that the next operation is a read from the data register
    • Read 16 bit data register

    Read second channel:

    • Write 0x01 to communications register:  This indicates that the next operation is a write to the configuration register
    • Set channel selection bits(CH2:CH0) in the configurations register to 0b001 to select AIN2 - AINCOM
    • Poll /RDY pin
    • Write 0x43 to communications register:  This indicates that the next operation is a read from the data register
    • Read 16 bit data register

    Was this followed?

    Since the AD7709 employs chopping and has extremely low offset and gain drift, calibration is not required.

    The input of the AD7709 performs chopping which removes the offset/zero shifts on the input, thus, there is no need to consider zero shifts.

    Thanks and Best Regards,

    Chris

  • Hi,

    i do the same process:

    1. reset device,

    2. set filter

    3. read channel AINCON-AINCON ( before you replay, we thought need read zero shift data )

    4. read channel AIN1-AIN2

    and the code is exactly same with yours.

    Now, i found change SPI bus frequency can improve correct chance, so i am testing this now.