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Comments on Driving ADC with opamp with large rails

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Driving ADC with opamp with large rails

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In the image below is an opamp buffer with +/- 15v rails connected to a 3.3v ADC. The input to the opamp is limited to 0-3v. There is no protection circuitry for the ADC.

Opamp buffer high rails to ADC

Is the risk of the opamp overdriving the ADC too great for no input protection to be used?

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General comments (4 comments)
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Here is a solution to protects very well the ADC from over-voltage spikes, avoid any simulation and design consideration, and that also protects the ADC from a possible negative output spike from the op-amp output at startup.

protect_ADC

For high frequencies, it may be necessary to put a "compensation capacitor" as explained in the answer of Olin.

Or just Shottky diodes to ground and the A/D supply. The Shottkys should kick in before the protection circuitry of the A/D. With a normal silicon junction drop, the internal protection circuitry might take most of the load.

The problem is that we know nothing about this ADC device. So, I consider it as a device sensitive to over-voltage, that may or may not have a protection circuitry against negative input voltage. Now, if it has no protection, the zener, with its 0.6V forward voltage should be sufficient in most cases (otherwise a Schottky may be connected in parallel as suggested by Olin). But if the ADC device has a negative input voltage protection, then even if it takes most of the load, it will dissipate at most 0.6 * 15/300 = 0.03 W. I can hardly imagine a protection circuitry that is not able to dissipate this power.

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General comments (1 comment)
General comments
Olin Lathrop‭ wrote almost 4 years ago

Or just Shottky diodes to ground and the A/D supply. The Shottkys should kick in before the protection circuitry of the A/D. With a normal silicon junction drop, the internal protection circuitry might take most of the load.