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MCU adc circuit protection
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hsn93:
Hello, im having three questions

I'm putting the information here that im not sure of so (if i said anything that is not correct please correct me) ..



i was looking to protect ADC of a microcontroller..



1. circuit:
i'm using industrial 1-5v sensor ... and looking into different solutions on the internet i came up with this



is this good ? can it be done better?


2. current limit:
the resistors (R2 top + R1) -> 560 ohm within the mcu characteristics according to this :


should i increase them to 2000 or something (would be more protected) ??



3. range?

Rbt (1K) to convert (1-5v) to (0.64-3.21v)

what is the "GAIN[0 to -vref]" what is the adc range here ?



Datasheet http://ww1.microchip.com/downloads/en/DeviceDoc/SAM-D21-Family-Datasheet-DS40001882B.pdf
ataradov:
1. Put diodes on the 3.3 V side and just do 2 resistor voltage divider. If this is an industrial application, you may need to do more analog filtering to get rid of the noise.
2. Maximum reference voltage is Vref = Vddana - 0.6V (3.3 - 0.6 = 2.7 V). ADC conversion rage is 0V to Vref, for the signal after it passed the analog gain stage. No idea why there is "-" in there.

So you need to further divide your voltage so the maximum is 2.7 V and set the gain to 1.  Note that you need to feed that reference level to  to VREFA.

A more convenient way  is to use REFSEL = INTVCC1 (1/2 Vddana) and have your signal maximum to be 3.3 V. Then you set gain to 1/2. This way you don't need a separate reference ,just make sure your Vddana is a reasonably clean supply. And you can have full 0 - 3.3 V signal mapped into full ADC scale.l  I personally prefer this variant.
David Hess:
I agree with ataradov; put the shunt protection diodes on the 3.3 volt side and combine the input resistor with the voltage divider.

Raise the impedance of the voltage divider as far as possible consistent with the input leakage of the ADC and accuracy requirements.  Add a filter capacitor across the ADC input consistent with the bandwidth requirements to limit noise.

If the impedance of the resistive divider cannot be make large enough to protect the ADC, then add a high impedance input buffer to drive the ADC.

JS:
Sorry to contradict you guys, you don't need the diodes to be connected to 3.3V but you do need a 3.3V zener (instead of the 5V connection). Even if you connect the diodes to the 3.3V rail the zener should be there to prevent excessing input current to elevate the rail voltage, or your µC would be feeding itself from the external source rather than the supplied current source, this could bring all sort of issues, like magic smoke escaping from the MCU or the PSU.

JS
David Hess:
It is not really a contradiction and there are a couple of ways to arrange the shunt protection.  Diodes are usually sufficient but a zener diode is better depending on the details.  Better than either is one or a pair of bipolar transistors with suitable bias networks.
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