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ADS8317: Negative voltage tolerance on signal input

Other Parts Discussed in Thread: OPA2376

The datasheet of this part shows that it has protection diodes on the signal inputs, which have a VF of 0.35V. The maximum ratings say "Input current to any pin except supply: -20mA to 20mA" It also says "Analog input voltage: –0.3V to VDD +0.3V"

I have seen similar absolute maximum ratings before where both maximum voltage and current were given, with a comment saying that some overvoltage is fine as long as the current doesn't get too high, but this isn't explicitly said here.

My input signal is in the range of 0V to 0.8V at the times I want to measure it, but between those times it regularly goes down to -0.8V. Currently I plan to protect the ADC from this negative voltage with an additional OPA2376 which I understand is tolerant of over/undervoltage (indicated in the way I mentioned above), working from a 0V/5V supply so that it can't produce negative voltage at the output.

However, this has the drawback that I can't measure all the way down to 0V, even though the op amp is Rail-to-Rail. I don't actually need to measure that far down as part of my application, but it would be great for easy calibration of system offset. The extra amplifier also adds its share of errors and signal delay (which isn't really a big issue, but less of that is always better :))

Do you think it's OK to put negative voltage to the analog inputs as part of normal operation, as long as I don't exceed the current limit? Given the RC network I already plan to place at the input, the current should be 1mA maximum under normal circumstances, and that only for 100µs each millisecond.

  • Hi Simeon,

    I would strongly recommend against driving the ADC inputs (over 300mV) below ground even with the 1mA current limit. Unless otherwise specified (as in the case of the OPA2376), on-chip input protection circuits are generally meant for sporadic ESD events that have a very specific energy dissipation profile, and not for being driven continuously in the way you are describing, which can potentially degrade long-term reliability.

    Using the OPA2376 on a single supply to drive the ADC is a great way to ensure that the ADC inputs remain positive. Yes, the amplifier will introduce errors, but I think achieving long-term reliability is a worthwhile tradeoff. Besides, the amplifier's input offset voltage would be negligible relative to that of the ADC (25uV vs. 1mV) as long as the op amp input common mode voltage is at least 1V below the positive supply (figure 23).

    Best Regards,
  • Hello Harsha.

    Thank you for clarifying this. I don't want to expose the part to unsafe conditions, so I will use an alternative solution - either the OPA2376, or a difference amplifier to shift the signal up from +-0.8V to VRef+-0.8V.