Simultaneous conversion by 2 ADCs

[acharya_satishb]

Hi folks,

In Kinetis MCU’s that have 2 ADC instances, if both ADCs are performing simultaneous conversions, can conversion process of one ADC interfere with conversion process of the other? Specifically, I see current spikes on REFL/H pins associated with charging of capacitor array at each successive approximation step as described in the ADC section of the Reference Manual. If both ADCs are converting simultaneously and charging its capacitor array independently, can current spikes of one ADC cause “incorrect” charging of the capacitor array of the other ADC? In other words, how does MCU ensure the charging of the capacitor array of one ADC does not collide with the other? Or, is it that the charging duration of the capacitor array of each ADC is long enough that even in case of a collision, the array gets charged to its desired value?

Do the ADC specs in the datasheet assume a single ADC operation? I’m not concerned with the small amount of crosstalk that might occur between the 2 ADCs but only with major issues associated with capacitor charging spikes.

For some NXP MCUs (e.g., MWCT101XS), there is an explicit recommendation in the datasheet to stagger conversions between the 2 ADCs. I do not see similar recommendation in any Application Note, Datasheet or Reference Manual for MK60DN256VLL10. Hence, my question above.

Thanks,

Satish

[acharya_satishb]

Thank you so much. This answers my question. Also, thanks for pointing to the ADC training material.

[Daniel-Aguirre · Accepted solution]

Hi,

Systems that have 2 ADC modules should be usable at the same time. There is a brief training that explains how using 2 ADC instances can even reduce current consumption when in low-power mode (ADC for Kinetis K Series MCUs, Section 6: FAQs, Question 4, rev 1.0). 

If the current spikes are a problem, then maybe there is not enough capacitance to support the charging and discharging. At the end, you will be having 2 inputs being sampled at the same time, the extra capacitance should be manageable. Still, staggering the Start of Conversion is recommended, to suppress possible noise coming from the switching of the ADC inputs.

We could say the same for PWM, you could start 2 PWM's at the same time, the MCU should be capable of, but at starting them at the same time, the noise generated by this signals will be doubling. By just staggering them a little bit, the noise will be reduced to a normal level.

Hopefully this helps with your question.