117x ADC Reference voltage - Only 1.8V?

[MathiasFriis]

Hi there,

I am currently checking out the MIMXRT117x-series for a future product and while investigating the LPADC-peripheral, I was a little confused that it appears that it is not possible to use 3.3V as reference voltage for the ADCs. My problem is that I want to use some legacy hardware, which outputs voltage in the region 0-3.3V.

If I understand this post [Re: MIMXRT1176 LPADC voltage reference scaling - NXP Community] correctly, the 117x-series does not support 3.3V reference voltage for the ADC, but only 1.0-1.8V.

I am aware that the CSCALE bit can be set in order to scale the sampled voltage down by a factor of 64/30. However, this effectively results in a loss of resolution, as the effective range of sampled values now becomes ~0-3500 instead of 0-4096, which of course is undesired.

In summary, I want to verify that it indeed is not possible to use 3.3V reference voltage for the ADC on the 117x-series before I start making HW changes.

I am currently using the MIMXRT1170-EVK, and we will be using the 1176-chip later on, I am unsure of whether this has relevance.

BR,

Mathias.

[MathiasFriis]

Thank you for the answer.

Follow-up question for clarification: Is it correctly understood that in order to take advantage of the full range of the ADC, I would need to use a reference voltage of (3.3V * 30 / 64 = 1.546V) while using CSCALE to scale down the analog input signal from 0-3.3V to 0-1.546V?

BR,

Mathias

[kerryzhou]

Hi @MathiasFriis ,

    ADC inputs can accept higher input voltages utilizing channel scale factor of 30/64. For example: if 3.3V input voltage required then scale factor must be enabled ADC reference must be set to 1.54V (only supported on VREFH).

   So, you are right!

Best Regards,

Kerry

[kerryzhou · Accepted solution]

Hi @MathiasFriis ,

   Confirmed, RT117X just support max 1.8V ADC reference voltage, but ADC can input 3V3v by utilizing channel scale factor of 30/64.CSCALE should set to 0:0 - Scale selected analog channel (Factor of 30/64)

Wish it helps you!

Best Regards,

Kerry