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ADS7830: ADS7830 application to monitor 12V

Part Number: ADS7830

Orginal link is here: https://e2e.ti.com/support/data-converters/f/73/t/710943?tisearch=e2e-sitesearch&keymatch=ADS7830

If you plan to monitor 3.3V rail directly then you can either use external reference >3.3V and VDD>3.3V to get correct ADC reading. The other way is to use resistive divide on 3.3V rail similar to one used on 12V rail to get the input within ADC full scale range

Hi 

If I want to use ADS7830 to monitor a voltage that was above vref scale (for example Vin >5V) , I would use a resistive divider, but I can't find any information of the impedance of analog input channel in ADS7830 datasheet, so how do I select the resistor of the divider to scale the voltage precisely?

regards

Chenhui

  

  • Hello, 

    Correct, you can use an external reference that is greater than 3.3V, and a VDD that is equal to or greater than the Vref voltage. This will allow to have an input of 3.3V

    I would like to point out that if you are connecting a voltage rail directly to the ADC input, to be sure to add some kind of protection to the ADC input, based on the fault conditions possible. example are a diode, or a current limiting resistor. 

    The ADS7830 is a SAR ADC, this means that is uses a switched capacitor circuit to sample input, this means that input impedance is dynamic and depends on the sampling rate. This is why the impedance is not specific in the datasheet.

    What sampling rate will you be using, and what is the voltage rail?

    Regards

    Cynthia

  • Hi

    In my applicationm I will monitor 6 rails of power, all of them have a max voltage of 7V. The MCU will scan these power rails no faster than 1 sample per second.

    Regards

    Chenhui

  • This is very doable.

    Can you be more specific when you say you will be scanning at no faster than 1sample per second

    Will you sample all six channels consecutively, than wait 1 second to sample all six again, or will each channel be sampled at 1 second

    This will help to know how long the input has to settle

    Regards

    Cynthia

  • HI 

    The sequence is that :  ch1 -->delay 10ms --> ch2 -->delay 10ms --> ch3 -->delay 10ms --> ch4 -->delay 10ms.......-->delay 1000- 9*10ms= 910ms --->ch1 -->delay 10ms --> ch2 -->delay 10ms ......

    Thanks

  • Hello,

    Apologies for the late response,

    You should be able to use resistor values in the tens to hundred of thousands. ie 10k, 100k

    For a 12V rail down to 2.5V, you can use R1 = 172K, R2 = 45K

    For a 7V rail down to 2.5, you can use R1=100k, R2 = 55k

    This based on a 100khz bus . If a faster bus is used, the value swill scale down

    Regards

    Cynthia