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ADS7828: ADS7828EB/2K5 SCH review

Lisa Xu
Intellectual 505 points
Part Number: ADS7828

Tool/software:

Dear TI Team:

We have ADS7828EB/2K5 in our design.

1. Could you pls review the SCH attahed, Thanks

2. Any layout suggestion or Tips for the PCB layout?

3. Is there any restrict for the Channel input series resistor? e.g. the series resistor value canno bigger than ? or others

8535.ADC SCH.pdf

Best Regards

Lisa

  • +1
    TI__Guru**** 180981 points

    Hi Lisa,

    We have a neat little tool that helps you choose the R and C value for the input to devices like the ADS7828: Analog Engineer's Calculator

    Using the calculator with the details in the datasheet for the sample and hold capacitor (25pF), the conversion time (6uS) and the ADC resolution (12-bits), we would recommend using no more than 4.4k ohms with a 500pF capacitor.  Other than that, I don't see any pull-ups on the SDA and SCL lines.

  • 0 in reply to Tom Hendrick
    Intellectual 505 points

    Hi Tom:

    Thanks for your reply, The pull ups(4.7K to 3.3V) for I2C is on the I2C mux side, it is not on this page.

    I will download this tool to Calculate, and I will feedback to you for update.

    For the pic you shared, the sample rate can be changed or not if we work on 100KHz/400KHz I2C in real application?

    Best Regards

    Lisa

  • 0 in reply to Lisa Xu
    TI__Guru**** 180981 points

    Hi Lisa,

    Yes - the sample rate can be changed.

  • 0 in reply to Tom Hendrick
    Intellectual 505 points

    Hi Tom:

    I have another question, 

    the 8ch use one ADC, does the sample rate 8K(SCL=400KHz) need be divided by 8  or not?

    maybe my direction is wrong,so just double confirm with you about this question

    Thanks

    Best Regards

    Lisa

  • 0 in reply to Lisa Xu
    TI__Guru**** 180981 points

    Hi Lisa,

    Yes - 50K is one channel.  Two channels is MAX 25k/CH, four channels is 12.5k/CH etc.

  • 0 in reply to Tom Hendrick
    Intellectual 505 points

    Hi Tom:

    Sorry,maybe my description is confused.

    I mean the value marked with red (from Datasheet)

    1. e.g. 8KHz is the throughput frequency when SCL=400KHz, and this 8KHz is the sampling rate in calculation Tool when SCL=400KHz, Right?

    2. Does I need use 8K/8=1KHz for each channel and put it in calculation tool to cal the relative Rin and Cin?

    3. And I donot know how to get this 8KHz when SCL=400KHz?

    Best

    Lisa

  • 0 in reply to Lisa Xu
    TI__Guru**** 180981 points

    Hi Lisa,

    for Q1 & 2, Yes, you can use the 8k/8 for the calculator.  For Q3, the conversion time is fixed at 6uS and then the delay though the I2C transmission (writing the address + command byte and then doing the reading of the conversion result) uses the rest of the time delta based on the SCL speed.