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Original question:

ADS7828: I2C 3.3V logic level compatibility with ADS7828 Vdd powered by 5V

ADS7828: ESD affect

Angus Huang
Prodigy 60 points
Part Number: ADS7828


Hi 

Questions as below

1)What is the function of the user option circuit?

2)Any ESD parts need in analog interface ? and the ESD parts will effect measurement result ?

3)Can the analog input measure the power supply directly? Need to add protection circuit to measure?

For example, when Vdd & Vref in are both 5V, can I directly measure the LDO output 5V voltage in CH0 ...etc

  • 0
    TI__Guru* 98055 points

    Angus,


    1. The User Option is just a set of RC filters. These filters are differential and common-mode filtering that are used for anti-aliasing. Usually, this includes the resistors. In the circuit, these are resistors R1-R8.

    2. I'm not aware of any specific ESD parts that are needed, but this may be dependent on the environment of your system. I have seen some systems that use Schottky or TVS diodes on the front end to prevent damage to the system. The additional leakage or capacitance may affect the measurement, but this is also dependent of the sensor that you are measuring.

    3. The range of the ADC is basically from 0 to VREF. To directly measure the device's supply, you would need to have a VREF larger than the supply, which is not possible. If the reference and the VDD are the same voltage, then you would always measure the full scale. To measure the supply, you would likely need a voltage divider and perhaps a buffer. For protection, do you expect that the power supply will have large spikes that require additional protection?


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 60 points

    Hi Joseph

    Thanks a lot for your help two questions for you

    1) I use VDD = 5V and external 5V for reference voltage also.

    my circuit is as below 

    Is there any need for component addition?
    These voltages are taken directly from the LDO, Is that OK?

    2) What else is there to pay attention to the ADS7828 design circuit, or what design guides are available can provide it to me

    Thanks a lot

    Angus 

  • 0 in reply to Angus Huang
    TI__Guru* 98055 points

    Angus,

    Sorry, I didn't notice this follow up question.

    The input current of the ADS7828 is listed as ±1uA. If you have a 1uA input current, this is a significant amount of current compared to the total through the voltage divider. This would cause a significant error in the measurement. You would either need a buffer or lower resistors in the divider to reduce this error.

    As for design guides, this is a rather simple device. I'm not aware of any design guides other than what is listed in the datasheet.

    Joseph Wu