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TMS570LC4357: Signal disturbance at ADC Channels Input - reg

Nandhakumar Palanisamy
Prodigy 65 points
Part Number: TMS570LC4357
Other Parts Discussed in Thread: TINA-TI

Hello,

in my project we are using TMS570LC4357 uC and we are using On-chip ADC channels to read the Analog input voltages, Input Voltage range is 0-24V, and there is a voltage divider circuit (ratio: 0.205) followed with a Op-amp voltage buffer circuit provided before ADC Input at uC. So the input to uC is 4.9V max. uC placed on a 10 layer PCB and the Voltage divider and buffer circuit provided in Another 8 Layer PCB. Both PCB's connected together by a Connector.

Issue Facing:

while we do ADC measurement, We are getting a Saw-Tooth pulse type noise pulse in ADC Channel input signal at between Voltage buffer o/p and uC input point. Refer the attachment.

1. we checked and found this noise pulse presents whenever the ADC sampling occurring, If we change the sampling time then the time difference between the noise pulse also changing.

2. if we increase the input voltage, the amplitude of the noise pulse also getting increase.

Due to this we are getting wrong ADC Value. Can anyone tell why this noise pulse occurring and how to avoid this....

Awaiting for your inputs.. thanks in advance.

  • 0
    TI__Guru 68920 points
    This makes sense if you think about it. When you switch the ADC to the channel you are sampling, it's essentially switching a capacitor onto the output of the buffer in your circuit, so you are changing the load and the output of the buffer is going to take some time to settle.

    The equivalent circuit for the load of the ADC input is in the datasheet electrical parameter section.

    You could do a first order approximation by hand or use TINA-TI to simulate the buffer on your board driving this load, and see if the result reasonably matches the result that you see on the bench.

    If it's not matching, or you need to change the circuit to make it settle faster then you need to do some design work in the analog domain.

    But if you are happy with the response, and just need to fix the issue w. the ADC result, you would need to increase the sample time parameter of that ADC channel to allow more time for settling. That is why we have this parameter programmable actually.. .

    Best Regards,
    Anthony