FRDM-KL26Z Single-ended ADC issue

[elisa_mejia]

Hi,

I am using a FRDM-KL26Z board to acquire data using ADC module. However, it doesn't seem to be working properly because of the voltage references. I've checked VDDA, VREFH, VSSA and VREFL and they are all 3.3V and GND, respectively, but the ADC result saturates at 1.05 V, and below that it doesn't have a linear response.

I am not sure what is happening but the code I'm using is very simple and has been shown to work properly in other FRDM boards. I'm even using the PE example published by @ErichStyger on the FRDM-KL25Z (https://mcuoneclipse.com/2013/02/10/tutorial-adc-with-the-freedom-board/comment-page-1/), but I'm having the same result. My first hypothesis is that the issue is due to hardware, but I'm not sure if there is anything else I'd need to check on the board.

Could anyone give me some light with this?

Best regards,

Elisa

[Robin_Shen]

Hi Elisa,

Please select FRDM-KL26Z and build the SDK_2.2.0_FRDM-KL26Z.

Then you can test the ADC examples.
I have test the frdmkl26z_driver_examples_adc16_interrupt on FRDM-KL26Z, connect the ADC signal at pin11 J4.
I am able to get the ADC result below 1.05V.

Best Regards,
Robin
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[elisa_mejia]

Dear Robin,

Thank you for your suggestion. I did what you say but I'm still having the same issues. The ADC value changes up to about 1 V and then saturates. Is there any other suggestion? Could it be that I need to connect some 0 ohm resistor in the FRDM board or something? I though the issue was with the voltage references for the ADC, but they seem to be OK.

Thank you,

Elisa

[Robin_Shen]

That example select kADC16_ReferenceVoltageSourceVref (external pins pair of VrefH and VrefL) as the ADC Reference voltage source.

If you have not modified the circuit of FRDM-KL26Z, the R10(0 ohm resistor) is connected by default.

Would you please measure the voltage of VREFH(Pin14) and ADC0_SE23(PTE30) by using multimeter?

[elisa_mejia]

Hi Robin,

Thank you for your suggestion. We have measured VREFH and VREFL and they give 3.3 V and GND, as expected.

Could I check anything else?

Best,

Elisa

[Robin_Shen]

Hi Elisa

Would you please show me more detail about this issue?
For example:
Whether you have modified the FRDM-KL26Z board.
Which project are you testing.
How did you connect external signal to ADC pin.( show me the circuit)

Why do you think the ADC result saturates at 1.05 V?
As the external voltage increases, does the value of the ADC data result register no longer increase?
What is the max value of ADC data result register?(tell me Conversion mode)

Maybe you can show me the value of registers during debug. And show the voltage value of the external signal measured by the multimeter.