# 117x ADC Reference voltage - Only 1.8V?

cancel
Showing results for
Show  only  | Search instead for
Did you mean:
SOLVED

## 117x ADC Reference voltage - Only 1.8V?

704 Views
Contributor II

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.

1 Solution
679 Views
NXP TechSupport

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

3 Replies
676 Views
Contributor II

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

656 Views
NXP TechSupport

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

680 Views
NXP TechSupport

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