S9S12P32 ADC input

[hyjung]

We are using the S9S12P32.

R1, R2 resistance is 50 kohm and R4 is 30 kohm.
At this point, when the input voltage TP1 rises to 35 V, the voltage at the TP5 (ADC input) increases by about 8 V, when the current in the R4 is about 60uA.

The input voltage of the ADC on the S9S12P32 is up to 5.5V, but you are wondering if the current capacity is low and can be actually damaged.

Because of ZD1, the voltage deviation of the ADC is large and unnecessary for voltage measurements, it is intended to be removed.

[danielmartynek]

Hello,
All the IO pins are internally clamped to VSSX and VDDX, or VSSA and VDDA.
The application must ensure that the injection current does not exceed the max specified current which is +-2.5mA (Single pin limit, Table A-7 #12 in the S12P reference manual).
Please refer to AN4731 Understanding Injection Current on Freescale Automotive Microcontrollers

Regards,
Daniel

[hyjung]

Thanks for your response.

I'd like to ask you some more questions.
ZD1 was removed from the attached circuit diagram and the input voltage of 35 V (TP1) was applied to confirm 8 V voltage at TP2.
If the injection current is limited to a maximum of 2.5 mA under this condition, the voltage of the TP5 will be clamped to 5.5 V.
If so, the actual injection current will be (8 V - 5.5 V) / 2.4 kohm = about 1 mA.
Therefore, is the maximum input power of one port 5.5V x 2.5mA = 13.75W?

I'd like to ask if you have any errors in your understanding of the above.

Thanks.

[danielmartynek]

Hello,

The calculation is about right.

But the injection current will have impact on the neighbor ADC channels.

Also, the additional Rs impacts the ADC accuracy.

Have you considered using a 6.1V ZD instead of this 4.7V ZD? 

This should effectively clamp the voltage.

Regards,

Daniel