KL17 Hardware ADC Voltage Range and Injection Current

[pac]

I’m using the KL17 in a design, and am using the single ended ADC. I need clarity on a couple of things.   The datasheet (page 32) states input voltage should be a max of VREFH. My VDDA = 3.3V, my Vref = 3.0V.  Does this mean my voltage must be limited to 3.0V, or does the datasheet mean it can only read to 3.0V, but is ok to VDDA (or VDDA+0.3V)?

Also, I see in the datasheet that it is stated that no diode to VDD, and thus no positive injection current number provided.  I have signals running to the single ended ADC pins through an OP amp, with diode protection.  (stripped down image attached) If my input is floating, the diode clamps and holds the voltage to 3.63V.  Measuring across the 1.74k resistor, I calculate 5nA of current. Is this acceptable approach, or do I risk damaging the micro?

Thank you.

[xiangjun_rong]

Hi, Patrick,

I see your scenario, the LM258 is powered by 5V, the maximum output voltage of the OP AMP is about 3.65V, which is greater than Vdd(3.3V) and vrefh(3.0V), you asked whether the Kl17 is possible to be damaged.

The data sheet states that the VADIN is greater than VREFL(GND) and less than VREFH(3.0V), obviously, the AMP output voltage(3.63V) is out of the range, I do not guarantee the KL17 can not damaged.

Can you connect a pull-down resistor on the AMP positive input pin so that the AMP outputs low when the input is float?

If you power the AMP with 4.3V power supply, it is okay.

Hope it can help you

BR

Xiangjun rong

[pac]

I didn't show the full circuit.  It doesn't have to do with the opamp itself, its if an input is left floating that the voltage could be clamped by the shown schottky diode to 3.63V.

So back to my first question, which you may have already now answered.  If my vref is 3.0V, and VDDA =3.3, you're saying the ADC input can only handle 3.0V and anything above that is risky? 

Thank you.

[xiangjun_rong]

Hi, patrick,

Generally, each ADC analog input pin has internal protection diode lest that the pin is damaged by ESD, if you limit the serial current to be less than 3mA, it is okay for high voltage. In other words, it is okay if the analog voltage is 3.65V, while you have used a 1.74K serial resistor in your circuit.

Hope it can help you

BR

Xiangjun rong