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ADS7038:Question about Input Current Spikes During Sampling in ADS7038

Daigo Kawaura
Prodigy 50 points
Part Number: ADS7038

Tool/software:

Dear TI Support Team,

I’m designing with the ADS7038 and have a concern about the instantaneous input current at the very beginning of the acquisition phase when the internal sampling capacitor is initially uncharged.

  1. At the start of sampling, does the ADS7038 exhibit a brief inrush current into the sampling capacitor? If we take VIN = 3.3 V and use the datasheet’s sampling switch on‑resistance RSW ≈ 150 Ω, is it reasonable to estimate the initial current as
    Iinrush≈VINRSW=3.3150≈22 mAIinrush​≈RSW​VIN​​=1503.3​≈22 mA?
    If this estimate is valid, how should it be interpreted relative to the 10 mA absolute‑maximum pin current—must instantaneous events also remain <10 mA, or are very short‑duration transients accounted for and acceptable?

  2. Are there recommended input network/driver practices (e.g., small series resistor with RC filter, acquisition‑time settings, or source‑impedance limits) to mitigate any adverse effects from this transient?

Thank you for your guidance.

Best regards,

  • 0
    TI__Genius 15700 points

    Hi Daigo,

    The 10mA limit is relevant while the AINx / GPOx pins are being overdriven. Per the abs max table, the minimum voltage the pins accept is GND – 0.3V, and the maximum is AVDD + 0.3V. 

    Operation below the minimum and above the maximum risks forward biasing internal ESD clamp diodes. In the case that the diodes are biased, current should be sufficiently limited as to not damage them. In this fault condition, even transient currents must be limited to below 10mA. 

    During normal operation within the input range, it is not a concern with a voltage source that is not normally forcing current. To avoid an overvoltage fault condition external clamp diodes are recommended. 

    Regards,
    Joel

  • 0 in reply to Joel Meraz
    Prodigy 50 points

    Dear Joel Meraz,

    Thank you very much for your previous response regarding the ADS7038 input current behavior.

    I would like to confirm my understanding:
    If the input voltage exceeds the absolute maximum ratings, then even transient input currents must remain below the 10 mA limit.
    However, if the input voltage stays within the specified operating range, is it acceptable for transient input currents to momentarily exceed 10 mA?

    If this understanding is correct, could you please explain in more detail why transient currents above 10 mA are acceptable in this case?
    I’m particularly interested in the internal mechanisms or design considerations that allow this behavior without causing damage.

    Thank you again for your support.

    Best regards,
    Daigo Kawaura

  • +1 in reply to Daigo Kawaura
    TI__Genius 15700 points

    Hello,

    Yes, frequently transient currents can exceed 10mA for nanoseconds at a time. However, this current is displacement current due to charge redistribution on the sampling capacitor, and it is designed for these transients. Current (save for some leakage current) does not physically pass through the dielectric and cause resistive heating sufficient to damage the internals. Damage can happen if sustained current is allowed to flow through the silicon junctions of the ESD cells during overvoltage conditions.

    This is why it is okay to connect a power supply directly to the ADC inputs while not damaging the device without any current limit, for all SAR ADCs within operating conditions. 

    Additionally, the RC filter and op-amp driver is usually to help the input at the ADC settle to full resolution accuracy. It is usually not for current limiting. 

    Regards,
    Joel