inductors and clean ADC supply

Discussion in 'Embedded Systems and Microcontrollers' started by albsure, May 7, 2014.

  1. albsure

    Thread Starter New Member

    Apr 1, 2012
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    0
    Hello World.
    OK, I'm generally a digital guy so let's get that out of the way.
    For one project, the goal is to provide a "clean power supply" to the ADC. In this case the ADC supply pin is both the analog reference and the supply to the ADC core itself. The chip runs on 3.3V.

    Problem: the ripple or noise on the 3.3V supply is not given.

    So, one implementation I've seen is to utilize an inductor (100uH) as well as typical bypass capacitors (0.1uF and 10uF).
    I want to make sure my digital results are calibrated correctly to the actual voltage at the ADC supply.
    Won't the inductor cause a minor voltage drop from the main supply to the ADC supply/reference pin? Given this is a DC voltage system, voltage drop across a 100uH or even 1000uH inductor should be minimal, right?
    TIA.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    Normally this would be set up as a Pi filter Cap-series ind-cap, the inductor would present its resistance only to the DC, the inductive reactance would have effect on the ripple portion.
    Max.
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    "Won't the inductor cause a minor voltage drop from the main supply to the ADC supply/reference pin? Given this is a DC voltage system, voltage drop across a 100uH or even 1000uH inductor should be minimal, right?"

    Any wire will have a voltage drop if their is current flowing through it.
    What is the resistance of the inductor?
    How much current is flowing through the inductor?
    Can you measure the ripple and noise on the power supply?
    What frequencies are you trying to eliminate?
    If you're doing this seat-of-the-pants and don't know what you want and don't measure what's going on and what your filter needs to do or is doing for you then you might as well just try something and hope for the best. If it is important you need to do it right. More inductance will attenuate noise and ripple more but likely also have higher resistance so more voltage drop.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    That's easy to calculate: it's just the total supply current times the DC resistance of the inductor (or E = I*R). Not knowing what your micro is nor what your inductor is I cannot pin this down any further.
     
  5. albsure

    Thread Starter New Member

    Apr 1, 2012
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    0
    Thank you for the replies. Some component selections and evaluation/experimentation are needed, and will be the best way to determine the behavior. The MCU is an NXP mbed platform, the ADC consumes around 2mA active. We'll see how it goes.
     
  6. John P

    AAC Fanatic!

    Oct 14, 2008
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    Let me ask what the analog input is being used to measure. Is it possible that the signal comes from something that could run on the same supply as the A/D itself? Because if so, everything could be ratiometric (if it were a strain gauge and instrumentation amplifier, for instance) and any deviations in the supply as seen by the processor would be matched by the input from the sensor, so this problem disappears.

    Even if you did have a perfectly smooth power supply, the way you have this set up means that the accuracy of the A/D is dependent on the accuracy of your 3.3V (unless the input is ratiometric). Is that OK? Generally power supplies aren't precision voltage sources.
     
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