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ADS1100: ADS1100

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Replies: 5

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Part Number: ADS1100

Hi team,

I am attaching the schematics of ADS1100A1IDBVT , please review it and let me know your inputs.
With regards,

Jayashree.

  • Jayashree,

    I don't have too many comments for the schematic. The ADS1100 is a moderately simple device and easy to use. The bypass capacitance is near 4.7uF, which is listed as the typical value from the datasheet. The input voltage divider is low enough to not be affected by the input impedance. The remaining pins are connected correctly.

    I did have two comments though:

    1. Make sure that GND and ANGND are connected directly. You don't want ground loops to affect the measurement.

    2. What is FB26? Sometimes there are inductors used to filter the supplies and small inductances shouldn't be a problem. However, large inductances can cause the supply to disrupt the operation of the ADC. The ADC will have digital currents, and with a large Ldi/dt, you can get large spikes in voltage. 


    Joseph Wu

  • In reply to Joseph Wu:

    Hi Joseph,

    Thanks for the reply.

    Joseph: The input voltage divider is low enough to not be affected by the input impedance. The remaining pins are connected correctly.

    Jayashree: What do you mean by this? Could you explain it to me in detail?

    Joseph: 1. Make sure that GND and ANGND are connected directly. You don't want ground loops to affect the measurement.

    Jayashree: I am connecting GND and ANGND through a ferrite bead, is that Ok? Attached image for your reference.

    Joseph: 2. What is FB26? Sometimes there are inductors used to filter the supplies and small inductances shouldn't be a problem.

    Jayashree: Fb26 I am using for as a filter circuit for the ADC IC supply power.

     

    With regards,

    Jayashree.

     

  • In reply to jayashree:

    Jayashree,



    1. The input impedance of the ADC is not infinite, so whenever you are making a measurement, you are loading whatever you are measuring with the input impedance of the ADC. If you look in the electrical characteristics table, you'll see the input impedance:



    If you use PGA=1, the input impedance is 2.4MΩ. Because of this, the 5.1kΩ resistor looks like 5.089kΩ (as the two are equivalently in parallel). With such a low measurement resistor, this is a small error. If you were measuring a divider of 1.5MΩ and 510kΩ, then this input impedance would cause a much larger error.

    2. I would not use the ferrite bead connection between GND and ANGND, and use a direct short. Normally, we recommend using a continuous ground plane, and consider if there could be ground loops in the layout based on the placement of devices. Again, if you have large amounts of digital currents in the digital ground returning through to GND, then this could cause problems.

    3. If you are using a single ferrite bead for the supply, then I don't think it would be a problem. The ADS1110 in itself wouldn't be enough digital current to cause issues.



    Joseph Wu

  • In reply to Joseph Wu:

    Hi Joseph,

    1. I think the ferrite bead Fb26 is of less impedance (470ohm) it is actually given for the pie section filter should I still reduce it?

    2. FB12 ferrite bead is given for isolation of digital GND and Analog GND.

    I am attaching the schematics, please tell me whether I should make any changes in it.

    ADC review.pptx

  • In reply to jayashree:

    Jayashree,

    My original comments still stand. I think the ferrite beads at the input of the supply should be fine, but I would not use ferrite beads to connect the analog and digital grounds.

    Joseph Wu