ADC Input protection in K22 series

[ngiussani]

Hi,

First of all, I'd been digging around the community and found no specific answer for my concern, so I'm afraid I have to ask for some help.

I'm implementing an application around a K22 series microcontroller (most likely the MK22FN128VLL10) We are trying to cut down parts, due to space/cost requirements. One of the most repeated circuits and most space consumming is the precission voltage clamping for the ADC input protection, so that's where we're aiming right now.

We are using 3.3V supply and 2.5V VREF, and we are not completely sure about the protection needed. It appears, form the datasheet, that we may have to clamp to VREFH to protect the input (despite the internal protections) and this may not be easy as we are using a voltage reference and not one with great source/sink capabilities. We are trying to implement a simple diode clamping scheme and doing so will end in either 3.3V + Vf or 2.5V + Vf on the input.

The main answer we are looking for: exceding this limit in an input is destructive or will only affect meassures?

[bobpaddock]

What exactly are you trying to protect against?

The 300 Ohm resistors need to be before the diodes to protect them as well.
The object is to keep the current low enough that it will not cause damage and high enough that the ADC works.

The diodes may leak with temperature extremes contributing to errors.

Depending on the structure of the ADC input impedance the resistor could cause issues.

In general it is better to drive a ADC input with op-amp.  Which is a conflict with keeping the parts count down.

A fault on one input could bleed over to an other input, depending on how the ADC-Mux is structured.

Google:

analog devices adc input protection

will bring up several design guides.

[ngiussani]

Hi, Bob. Thanks for your answer.

Let's assume this:

• VREFH = 2.5V
• Input voltage, before external clamping diodes, is 3V. External clamping diodes will not protect against this voltage. 300Ohm resistors are placed to protect internal diodes. External diodes will protect at approximately 3.6V with the example circuit.

Is this condition destructive to the input? It's not clear from the datasheet if it is or not.

P.S.: Google was (and always is) my first option, but there is nothing that suit my application and requirements. Of course, as I stated in my original post, I can use a precission Op Amp solution, but is the one I'm trying to avoid.

[mjbcswitzerland]

Hi

ADC protection/circuit design may be an important topic so it is worth carefully investigating possibilities.

See the following for a practical case where the ADC operation really failed in the field after a certain amount of time: https://community.nxp.com/message/593170?et=watches.email.thread#593170 

Regards

Mark

[ngiussani]

Thank yo, Mark

I read the post you mentioned prior to posting. Like many other posts in here, it's unanswered and unresolved. A shame, really, because I think there some very important issues being treated in these threads.

I'll wait for any help.

[armandonovello]

Hi Kerry,

What would be the solution recommended by NXP for this case to avoid permanent damage to the MCU?
Could you suggest a circuit for example?

Best regards

Armando Novello

[ngiussani]

To clarify and elaborate, we are thinking in something like this

Will it protect the inputs? Or we need to clamp to VREFH?

[kerryzhou]

Hi Nicolas Giussani,

    From the datasheet of K22, we can get this information:

  Your VREFH is 2.5V, so you should make sure your adc input is in range[0, 2.42v] in 16-bit differencetial mode, other mode, the input adc voltage should in the range [0,2.5v], otherwise, your adc input pin maybe damaged.

Wish it helps you!

If you still have question, please let me know!

Have a great day,
Kerry

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