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I have a couple questions related to protecting my instrumentation amplifier and the ADC of my MCU.

The circuit is as shown in the image, with the input to the in-amp (an AD8223) being a very weak signal that occasionally spikes to 20VPP. The output reference pin of the in-amp is set at 1.7V (although it's not shown in the schematic), and as one might expect my microcontroller's ADC can only handle 0-3.3V.

  1. My somewhat-rusty understanding is that R1 will limit the current going through the first two zener diodes (D1,D2) so R2 and R3 aren't necessary. Is that correct or am I missing something?

  2. How can I calculate the output current from the in-amp to determine if R4 is necessary to protect D3 and/or the ADC?

  3. Are there any other input protection elements I should add?

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What you will likely find is that a simple resistor in series between the InAmp output and the MCU input is all that will be necessary to protect the MCU. Look up the current that can be put into the MCU input in the absolute max rating table for the MCU.

Then, you can probably run your InAmp from a +5 volt rail because, running it from 3.3 volts means the maximum level it can produce is a 150 mV short of its Vcc rail. Given that you could use a current limit resistor between InAmp and MCU input, that will offer any over-voltage protection from the InAmp.

Regards running the InAmp from a negative rail (as opposed to 0 volts) is tenuous given that the InAmp output can swing down to within 10 mV of the negative rail (or ground).

In addition, the InAmp data sheets says this about input over-voltage protection: -

Internal supply referenced clamping diodes allow the input, reference, output, and gain terminals of the AD8223 to safely withstand overvoltages of 0.3 V above or below the supplies. This is true for all gains, and for power-on and power-off. This last case is particularly important because the signal source and amplifier can be powered separately. If the overvoltage is expected to exceed this value, limit the current through these diodes to about 10 mA using external current limiting resistors.

So, by careful choice of series current limiting resistors on the InAmp input, you can avoid the need for zeners.

My somewhat-rusty understanding is that R1 will limit the current going through the first two zener diodes (D1,D2) so R2 and R3 aren't necessary. Is that correct or am I missing something?

Answer: Missing something

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  • \$\begingroup\$ Thank you. "Answer: Missing something": Can you give me a link or a search term to follow up on? Regarding the rest of your reply, I'm stuck using a 3.3V rail and I do need the negative rail as the signal is sometimes below the -150mV limit of the in-amp. Given that I do at least need D3. Do you have a direction you could point me in to selecting R4? \$\endgroup\$ Aug 18 '20 at 17:37
  • \$\begingroup\$ Missing something as in you were missing the trick of putting a current limiting resistor between the InAmp and the MCU as I said in my answer. Current limiters all round I would say (or have said). \$\endgroup\$
    – Andy aka
    Aug 18 '20 at 17:43
  • \$\begingroup\$ @kattmannsix if you are done with this answer, can you formally accept it or raise another comment for whatever clarification you need. \$\endgroup\$
    – Andy aka
    Feb 16 '21 at 15:09

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