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Thread: Does the Teensy 3.2 have a ratiometric ADC?

  1. #1

    Does the Teensy 3.2 have a ratiometric ADC?

    Hey, I am trying to temperature through an NTC thermistor accuracy, current I am using an Arduino UNO, but its ADC does not have enough bits for the accuracy I am looking for. This has a bit 3 more bits. My question is, the ADC on the Teensy 3.2 ratiometric?

  2. #2
    Senior Member
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    I am not sure about ratiometric but you can use these on Teensy 3.x
    Code:
    void setup() {
      // put your setup code here, to run once:
      analogReference(EXTERNAL); // set analog reference to EXTERNAL-INTERNAL ref
      analogReadRes(16); // Teensy 3.x   set ADC resolution to this many bits (Usable 13bits) (default 10 bits)
      analogReadAveraging(16); // average this many readings
    }
    
    void loop() {
      // put your main code here, to run repeatedly:
    
    }

  3. #3
    Thank you, but enabling external ref just allows for a more precise voltage correct? still not ratiometric?

  4. #4
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    For 3.6 I found with 3.3V ref FS will be 3V3 and with 1.1V ref FS will be 1.1V .
    This is ratiometric. But i dont know about accuratie if you give him much less than 1V ref......

  5. #5
    Quote Originally Posted by Wibbing View Post
    For 3.6 I found with 3.3V ref FS will be 3V3 and with 1.1V ref FS will be 1.1V .
    This is ratiometric. But i dont know about accuratie if you give him much less than 1V ref......
    Do you think the 3.2 ADC's is ratiometric? Is there anyone I can contact from the company?

  6. #6
    Senior Member+ KurtE's Avatar
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    Sorry I am not an expert at all in Analog nor work for the company... Nor know what ratiometric means or implies... Other than there is a ratio...

    But if it were me, probably the first thing I would do is look at the data sheet for the different processors, which you can find up at: https://www.pjrc.com/teensy/datasheets.html

    But other alternatives if these don't work, is to find an external device that does the analog conversions for you and you can retrieve the values using something like SPI or I2C...

    Example: https://www.adafruit.com/product/1085

    But again I don't know enough to know how well these will work for you...

  7. #7
    Senior Member DD4WH's Avatar
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    If you could explain to us what you mean precisely by "ratiometric", we could be better in answering your questions.

    If "ratiometric" means to you: "The ADC output value is proportional to the voltage at the input of the ADC", then I would say yes (given the maximum and minimum input voltage limits of the Teensy 3.2 ADC)

  8. #8
    Quote Originally Posted by DD4WH View Post
    If you could explain to us what you mean precisely by "ratiometric", we could be better in answering your questions.

    If "ratiometric" means to you: "The ADC output value is proportional to the voltage at the input of the ADC", then I would say yes (given the maximum and minimum input voltage limits of the Teensy 3.2 ADC)
    I think your interpretation is correct. I mean does the ADC take in the output voltage, and use it as its input voltage. That way if the voltage dropped for some reason to 3.2v the ADC would scale its values to match the incoming voltage.

  9. #9
    Quote Originally Posted by KurtE View Post
    Sorry I am not an expert at all in Analog nor work for the company... Nor know what ratiometric means or implies... Other than there is a ratio...

    But if it were me, probably the first thing I would do is look at the data sheet for the different processors, which you can find up at: https://www.pjrc.com/teensy/datasheets.html

    But other alternatives if these don't work, is to find an external device that does the analog conversions for you and you can retrieve the values using something like SPI or I2C...

    Example: https://www.adafruit.com/product/1085

    But again I don't know enough to know how well these will work for you...
    Thank you, but for this project, I am not allowed to use an external ADC.

    The datasheet really helped, thank you!

  10. #10
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    Ratiometric means: the output code is proprtional to the RATIO BETWEEN THE INPUT VOLTAGE AND THE REFERENCE VOLTAGE.

    Example: If we use the ExcitationVoltage of a MeasuringBridge as reference and the Output of the Bridge as ADC-input, all errors of the reference will be eliminated by divition to 1.

  11. #11
    Quote Originally Posted by Wibbing View Post
    Ratiometric means: the output code is proprtional to the RATIO BETWEEN THE INPUT VOLTAGE AND THE REFERENCE VOLTAGE.

    Example: If we use the ExcitationVoltage of a MeasuringBridge as reference and the Output of the Bridge as ADC-input, all errors of the reference will be eliminated by divition to 1.
    Thanks! that is what I ment.

  12. #12
    Senior Member+ Theremingenieur's Avatar
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    It's just that the limits for Vref are 1.25V to 3.3V (c.f. data sheet). Within these boundaries, the ADC is definitively ratiometric. That's why many users use the 3.3V Vdd reference when reading potentiometers whose high end is also connected to 3.3V. This eliminates reading errors from varying Vdd.

  13. #13
    Ratiometric means that the sensor output is proportional to its excitation voltage and that the excitation voltage is identical to the ADC reference. Do that and the absolute value of the excitation voltage falls out of the equations and doesn't effect accuracy at all.

    Otherwise you need nice stable and accurate sources for both the excitation and ADC reference.

  14. #14
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    It seems that this strange "ratiometric DAC" is no more than a subset of a classical MDAC (multiplying DAC) : https://www.analogictips.com/what-is-a-multiplying-dac/ or am I wrong ?

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