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Thread: AnalogIn + voltage divider (Teensy 3.2)

  1. #1

    AnalogIn + voltage divider (Teensy 3.2)

    Hi everyone,

    I am currently working on voltage dividers on Teensy 3.2.
    In order to be able to calculate the R1, I would have to know how much current can flow at the Analog IN pin (I max?)
    with the Teensy 3.2 (at 3.3V)?


    For the AnalogOut I found a value of 10mA in the specifications.
    Unfortunately, with Analog IN there is no information (or do the 10mA refer to both, In + Out?)

    I would appreciate your information!


    Greetings from Germany

  2. #2
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    From your description, it is difficult to understand what you are trying to do. This thread may help... https://forum.pjrc.com/threads/28722...-ADC-impedance. A.C. alternating signals are also a different consideration to D.C.

    When dealing with signals above the "Vin" voltage, or signals that can go less than "GND" you may need to take additional precautions (fast diode clamping) depending on the "risk". You must ensure that any pin is never taken outside the pos/gnd supply limit, not even for a split second.

    The Application Note referred to can be found here...https://www.nxp.com/docs/en/application-note/AN4373.pdf.
    Last edited by TelephoneBill; 06-13-2020 at 11:41 AM.

  3. #3
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    Also... for a voltage division, you will need R1 and R2, where R1 is between signal source and pin, and R2 is between pin and GND. The division is then R2/(R1+R2)*Vsig. For a simple 3:1 division, R1=20K and R2=10K would work if the sampling is not too fast. Best thing is to test it with a known input and see what value the ADC gives you.

    Beware this only works for signals between 0v and +10v (for 3v3 supply). If its A.C. then you need a more elaborate config.

  4. #4
    Senior Member oddson's Avatar
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    My amateur understanding of input pins it they are like op-amp inputs... very high input impedance means you can mostly ignore it provided your output impedance in on the order of KOhms.

    The risk if from Voltage. Some Teensy 3.x have 5 volt tolerance (incl. 3.2) but in general you need to ensure the voltage doesn't exceed 3.3 volts and the input impedance will keep the input current below the max if you do.

    Signal conditioning is very specific to the signal. Details about that would get you better advice.

  5. #5
    Hi everyone,

    First of all thanks for your answers !


    of course the voltage divider consists of R1 and R2 - sorry if I have expressed myself somewhat unclear ...

    R1 is on + 3.3V DC, the other side on PIN A1. From pin A1 it goes via R2 to GND.
    In my case, both resistors are the same (currently 3.3K).

    With 3.3K at 3.3 volts I am currently at max 1 mA.

    Somewhere in the Internet there was a information from I max = 1.65mA on the Analog IN pin. Thereafter, Rges
    should not be less than 2000 ohms.
    However, I am no longer sure whether this is true or whether it refers to a different board ...

    With this information and a little extra security, I ended up at 3.3 K for R1 and R2 so that the Teensy on the Analog IN was not constantly at the limit.


    Since R2 will soon consist of 24 resistors (initially a 3.3k), it could then be difficult with the resolution and interference.

    Midi note ON / Off commands should then be triggered with a resistance measurement.
    To achieve this, R2 to R25 can be bridged with GND so that Rges changes accordingly.

    However, the previously identical resistances do not necessarily have to be 3.3 K if the Analog IN PINs
    also allow significantly smaller values (this would be very helpful in avoiding interference ...).

    Hence the question of how much current can flow max and permanently at the Analog IN with Teensy 3.2 ...

    Who knows the details here?


    regards

  6. #6
    Senior Member oddson's Avatar
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    I'm more baffled than before... it sounds like you will be reading a constant 1.65 volts ...where is the signal part?

    Almost no current will actually flow into your Teensy... but if the voltage is exceeded you will fry it.

    See Paul's reply here: https://forum.pjrc.com/threads/28722...-ADC-impedance

  7. #7
    Quote Originally Posted by oddson View Post
    I'm more baffled than before... it sounds like you will be reading a constant 1.65 volts ...where is the signal part?

    Almost no current will actually flow into your Teensy... but if the voltage is exceeded you will fry it.

    See Paul's reply here: https://forum.pjrc.com/threads/28722...-ADC-impedance

    Hi Oddson,

    I think there is a misunderstanding here!


    The voltage divider with only 2 resistors (R1, R2, each 3.3K) was only one example. It is completely clear that 1.65 V is measured
    (1 mA flows through the AnalogIN).

    The aim is to later replace R2 with 24 individual (equally large) resistors in order to be able to trigger midi notes with the changed Rges
    values of the voltage divider.


    I want to be able to calculate how small R1 can be at the most so as not to overload the AnalogIN current side.
    Without Imax for the AnalogIN, R1 cannot be calculated ...

    My previous information was 1.65 mA or 2000 Ohm - although I'm not sure if that's true.

    Especially since the specifications of Teensy 3.2 state that the AnalogOut can deliver 10mA.
    So it is reasonable to assume that the value may also apply to the Analog IN?

    Hope I could solve the confusion a bit?



    regards

  8. #8
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    I'll help by stating your question better:

    What value resistors do I need to use on a teensy analog input voltage divider to assure that I have sufficiently low source impedance?

    I use 1000 and 316 ohms to divide by 4.

  9. #9
    Senior Member oddson's Avatar
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    At the top of a pot used as a voltage divider there is no significant resistance to the pin from the Vcc. The current is almost entirely going out the bottom of the divider and only the smallest trickle leaks through the input pin.

    For fixed switched resistors (which I finally get you are talking about) the less resistance the faster it will charge the input capacitance but the more current you will draw. You don't need to worry about too small.

    I think you're up the wrong tree but I'm not expert enough to say with certainty.

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