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I want to use an AD7124-8 ADC in a project:

I was wondering how to protect the analogue inputs from reverse polarity. According to the datasheet inputs can tolerate at most 0.05/0.1 volts of reverse voltage:

Maximum Tolerable Analogue Input Voltage

Thus, clamp diodes cannot offer any protection here. Any suggestions on how to resolve this issue?

P.S. Reverse voltage is at most 15 volts.


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Wouldn't an external reverse polarity protection diode do the trick here? – Krunal Desai Feb 21 at 8:50
up vote 7 down vote accepted

The datasheet is a bit misleading here: it describes the voltage limits you've quoted as "absolute", but it's as opposed to differential rather than the safety limits of the device.

The safety limits are provided on page 13, and we see that the analog inputs can tolerate up to 0.3 V below ground, and can tolerate an injection current of up to 10 mA. This almost certainly means there are internal clamping diodes, so you only need to add a current-limiting resistor between the external input and the ADC.

The larger the resistor the better, but at least 15 V / 10 mA = 1.5 kΩ is necessary. Keep in mind, though, that the resistor effectively adds to the impedance of your source, so bandwidth will be affected.

Perhaps more important than the bandwidth, the ADC inputs have a leakage current, which will be multiplied by the source impedance to produce a voltage error. Since the ADC is differential, the error is based on the difference between the leakage currents, typically 1.5 nA in full-power mode. With 1.5 kΩ, this produces an error of over 2 microvolts.

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The resistor shouldn't be too large because of the input current (approximately 1 nA). – biggvsdiccvs Feb 21 at 9:30
Good point—I hadn't looked at the input current specs. I'll mention that in my answer. – Abe Karplus Feb 21 at 9:31
Thanks for the great response. This completely resolved the issue. I don't know how I missed the absolute Maximum ratings on p. 13 :) . BTW, bandwidth isn't an issue here. I'm reading low frequency signals. – hadez Feb 21 at 10:26

See p.13 of the datasheet, Absolute Maximum Ratings. The max reverse voltage is -0.3V, so a Shottky diode should suffice. Perhaps the circuit already has clamp diodes an this is where the value comes from, although I don't see any mention of them in the datasheet.

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Yes you are correct, I missed the -0.3 v part. – hadez Feb 21 at 10:28

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