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TM4C1294KCPDT: ADC noise with internal reference

Stepan Mihalin
Prodigy 20 points

Part Number: TM4C1294KCPDT

Hello.

I connect ADC pin to ground and got the measurement graphs of the measurement ADC values

I do same experiment with EKS-LM4F232:

Nosie about 10-15 counts. Why? For my LM3S5R31 i see ADC noise about 2-3 counts (with internal reference).

  • 0
    Guru 27665 points

    A few notes:

    • Read the specs, particularly the noise specs (translate db to bits and bits to counts)
    • By eye, your noise is probably 5 counts or so. It's certainly not 15
    • Your reference is the analog power supply
    • Your result will depend critically on your input circuit (including parasitics)
    • For constant noise 15 counts on the on a TM4C1294KCPDT (11.99bits) is better than 2 counts on the  LM3S5R31  

    TI has some good workshop material on care and feeding of A/Ds (search for the tag bookshelf).

    Robert

  • 0 in reply to Robert Adsett
    Prodigy 20 points

    - "For constant noise 15 counts on the on a TM4C1294KCPDT (11.99bits) is better than 2 counts on the  LM3S5R31"

    Why?

    For 10-bits ADC (LM3S5R31): 2 counts / 1024 = (1 / 512)

    For 12-bits ADC (TM4C129KCPDT): 15 counts / 4096 ~ (1 / 256)

    (1 / 256) > (1 / 512) => noise more for TM4C.

    - "Your result will depend critically on your input circuit (including parasitics)"

    I just connect ADC input to microcontroller ground

    - "By eye, your noise is probably 5 counts or so. It's certainly not 15"

    I mean maximum value

    - "Read the specs, particularly the noise specs (translate db to bits and bits to counts)"

    I see maximum offset error for TM4C 15, for LM3S 20.

    I calculate noise for TM4C with help datasheet, noise must be less than 15 counts

    I taked measured current graphs for TM4C and LM3S, pins are connected to ground

  • 0 in reply to Stepan Mihalin
    Guru 27665 points

    Stepan Mihalin said:

    - "For constant noise 15 counts on the on a TM4C1294KCPDT (11.99bits) is better than 2 counts on the  LM3S5R31"

    Why?

    For 10-bits ADC (LM3S5R31): 2 counts / 1024 = (1 / 512)

    For 12-bits ADC (TM4C129KCPDT): 15 counts / 4096 ~ (1 / 256)

    (1 / 256) > (1 / 512) => noise more for TM4C.

    See bolded observation. 4080 counts noise free > 1021 counts noise free.

    Stepan Mihalin said:

    - "Your result will depend critically on your input circuit (including parasitics)"

    I just connect ADC input to microcontroller ground

    And such a measurement will depend critically on how you connect to ground. How long is the connection, how is it routed, do you have a capacitor on the input. etc.., etc...

    Stepan Mihalin said:

    - "By eye, your noise is probably 5 counts or so. It's certainly not 15"

    I mean maximum value

    That includes offset

    Stepan Mihalin said:

    - "Read the specs, particularly the noise specs (translate db to bits and bits to counts)"

    I see maximum offset error for TM4C 15, for LM3S 20.

    Nope. That would appear to be the offset not the noise. Noise is in the next section of the datasheet.

    Stepan Mihalin said:
    I taked measured current graphs for TM4C and LM3S, pins are connected to ground

    I have no idea what you are doing here. You haven't explained what current you are measuring, how you are measuring it or why it would matter.

    Robert