Comments on Driving ADC with opamp with large rails
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Is the risk of the opamp overdriving the ADC too great for no input protection to be used?
I would certainly say yes but, this can usually be easily solved by using a current limit resistor in the feed line to the ADC. Most ADCs specify a maximum current that their inputs can take. This is an overdrive level and doesn't normally apply when input signals are correctly bounded. $$$$ But, on power-up situations the op-amp output may be able to deliver more than 25 mA at a voltage much larger that 3 volts so, a resistor is a simple and effective counter-measure. $$$$ If the maximum ADC input current spec is 5 mA, then what resistor value will prevent more than 5 mA when 15 volts is applied to the ADC input? You can assume that the ADC input diode protection will try and clamp to a little above 3.3 volts so assume 3.5 volts. It then boils down to: -
$$\dfrac{15\text{ volts} - 3.5\text{ volts}}{R} = 5 \text{ mA}$$
So, R = 2.3 kΩ.
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