TMS320F280039C-Q1: ADC sensing error

Aki Li

Tool/software:

Customer used 2 pcs F280039C-Q1 in the OBC project and found one of them showed large temperature sensing error.

After further checking,

They found the voltage drop across the 20kohm pull up resistor was different between 2 devices, in the below circuits (the pull down resistor was removed during the test)

1 device showed nearly 0V with ADC pins (B0/A8), while the other device showed 0.02V drop across the 20kohm with ADC pins(A0/B12). It seemed the input current was different. Could you please help check the possible reasons?

5 months ago

0 Hadi Moradisizkoohi 5 months ago

TI__Expert 6650 points

Hi Aki,

Is voltage drop equal if the same pins (A0/B12) are used for ADC?

Regards,

Hadi

0 Aki Li 5 months ago in reply to Hadi Moradisizkoohi

TI__Genius 9205 points

Hadi,

We identified the reasons for this issue now. After we changed the ADC trigger source PWM frequency from 60kHz to 1kHz, and there was no voltage drop.

Could you please help provide detailed explanation for this and suggestions to address it in future design?

0 Hadi Moradisizkoohi 4 months ago in reply to Aki Li

TI__Expert 6650 points

Aki,

Considering the input circuit of the ADC, combination of source resistance and the capacitance Cp can acts like a voltage divider in high frequencies. So, there is a voltage drop at high-frequency operation. To resolve the issue, you should make the source resistance RS smaller. Using a buffer in front of the ADC is another solution.

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TMS320F280039C-Q1: ADC sensing error