ADC resolution issues on RT1170.

[Pavankumar1]

Hi, I am working on the ADC in the MXRT1170 controller. When I set the reference voltage to 1.8V, for a 1.26V input to the ADC, I get an ADC value of 2800, which is correct when converted to voltage. However, when I set the reference to VREF and set CSCALE to 0, for the same 1.26V input, I get an ADC value around 1300. When I convert this ADC value to voltage, I get a result that is about 0.2V less compared to when I use 1.8V as the reference voltage.

[Habib_MS]

Hello @Pavankumar1,

The CSCALE bit, reduces the selected ADC analog channel input voltage level by a factor of (30/64). And it does this by increasing the VREF by multiplying it by the factor (30/64)

For example: if VREFH is 1.8V and CSCALE = 0, then the ADC input voltage can be up to 3.84V because 1.8*(64/30) =3.84. Consequently, the value that will be in your ADC value can be obtain of the next formula:

To obtain more information you can see the chapter 87.6.1.3 called "Analog input pins" in the RM.

Also, if you experience any issue, do not hesitate to let me know.

BR

Habib.

[Pavankumar1]

Hello,@Habib_MS

I am using the ADC in the RT1176 controller with a 1.8V reference voltage. I am able to read values above 15.2mV correctly, but values below 15.2mV are not read accurately. Could you please help me understand what might be causing this issue and what the minimum value is that the ADC can reliably read?

Thank

[Habib_MS]

Hello again @Pavankumar1 ,

The ADC by default has a variability of the values taken, in this case the gain error typically is -0.16 %FSV according to the table 76 in the datasheet. Also, the ADC has another variability because this range is near of 0 V, this variability is typically +-0.01%FSV as mentioned in the data sheet.

This happens because the response of the ADC ceases to be linear as it approaches one of the limits of its range, Also, around 0 V the noise affects more to the conversion.

Also, if you experience any issue, do not hesitate to let me know.

BR

Habib.