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ADS7961-Q1: Powe up sequence, CHX earlier than Vx

Charles (KAT) Zhang
Intellectual 1595 points
Part Number: ADS7961-Q1
Other Parts Discussed in Thread: ADS7961

Tool/software:

Dear team,

My customer has an application where device on CH1 will be powered up earlier than ADS7961

I see in D/S MAX CHx is VA+0.3, so I believe we are not allowed to do so?  we need to be the first device to be powered up?

Looking forward to your reply,

Charles

  • 0
    TI__Genius 14270 points

    Hi Charles,

    Yes, a condition where the input is present before the supply should be avoided, or protected against with external protection diodes.

    If the voltage on the analog input does exceed the analog supply by 0.3V, then it should be ensured that the input current to the ADC is limited to below 10mA. To ensure device longevity, best practice is to keep it below 1mA. 

    Regards,
    Joel

  • 0 in reply to Joel Meraz
    TI__Intellectual 1595 points

    Hi Joel,

    How should they ensure the input current is lower than 1mA? in input current depends on the load, where now the load is ADS7961 internal circuit.  for example, if they have a voltage of 1V on ADC CHX, then if out internal reistsance is 1K then we can have a 1mA input current.  What is our internal resistance when VA = 0?

    Thank you

    CHarles

  • 0 in reply to Charles (KAT) Zhang
    TI__Genius 14270 points

    Hi Charles,

    The equivalent input circuit to the ADC is modelled by a switched RC circuit, so the current it allows at a given moment will vary depending on the charge of the sampling capacitor. 

    The maximum input voltage can be divided by the equivalent input resistance to get max current. Consider a 5V input, with a 10kΩ series resistor between the input and ADC analog input pin. Maximum current would then be limited to 5V/10kΩ (0.5mA).

    Essentially, consider the Thevenin equivalent resistance between the input an ADC input pin, including source output impedance, ADC input resistance, any series resistance, and parallel resistance.

    Regards,
    Joel

  • 0 in reply to Charles (KAT) Zhang
    TI__Genius 14270 points

    And just to be clear, here is the section of the datasheet describing the equivalent input resistance.

    Regards,
    Joel