Allowed Voltages on A/D Converter pins?

[Laartoor]

I am a bit confused about the allowed voltages on the ADC pins of the Kinetis K.

a) In section 4 of the data sheets, we have the pin ratings: Vdio is defined as -0.3 to 5.5 volt, and Vaio as -0.3 to Vdd+0.3 volt, where Vaio is for the pure analog pins and Vdio for mixed pins.

b) In section 6 we have the ADC spec, and it states that Vadin must be between Vref low and Vref high.

When I now have a voltage on a pin that satisfies a), but falls outside b), what will happen:

- may I have the pin connected to an analog function, even it if falls outside Vaio?

- may I start an A/D conversion?

The reference manual already tells me that the ADC will clamp values outside Vref low and Vref high to the lowest/highest possible conversion result.

[Paul_Tian]

Hi, Laartoor

If analog input voltage is outside of range from VREFL to VREFH, the value converted will be incorrect. Yes, you can start a AD conversion, but the value you get is incorrect.

Hope my reply can help you.

Best Regards

Paul

[Laartoor]

Thank you for your response, Paul.

So if I would connect an analog pin with a digital function to a constant 5.4V, and then repeatably do a conversion on that pin with say 10K samples per seconds, the ADC would remain operational even after doing this for a year?

The reference manual states:

For proper conversion, the input voltage must fall between VREFH and VREFL. If the input is equal to or exceeds VREFH, the converter circuit converts the signal to 0xFFF, which is full scale 12-bit representation, 0x3FF, which is full scale 10-bit representation, or 0xFF, which is full scale 8-bit representation. If the input is equal to or less than VREFL, the converter circuit converts it to 0x000. Input voltages between VREFH and VREFL are straight-line linear conversions. There is a brief current associated with VREFL when the sampling capacitor is charging.

So I could rely on the conversion above to always return 0xFFF, if I configured it for 12-bit conversion?

[perlam_i_au · Accepted solution]

Theoretically you can use an input of 5.4v for your ADC and as long as your VREFH has been set in a lower value than the input, yes, your ADC result will be 0xFFF if you have 12-bit ADC.

Here is were I would like to ask you about which micro controller are you using? this because Vdd limits.

In Kinetis E Series MCUs Vdd=6v, while in Kinetis K Series MCUs Vdd=3.8v; please consider that VAIO (Voltage for analog pins, those which does not have a general purpose I/O port function) have to be min=-0.3v and max=Vdd+0.3v, and if you are using a voltage value above of the limit this could damage or at least decrease the lifetime of your device.

Have a nice day,
Perla Moncada

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[Laartoor]

Thank you for the response, Perla.

We use a 3.3V Vdd supply, and to clarify, 3.3V would already be higher than the expected reading during normal operation, but it is possible for it to rise to 5V or bit higher in extreme circumstances. We would then switch to a reduced functionality mode, but we cannot afford the CPU to become damaged.