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Prop2 Analog Input range — Parallax Forums

Prop2 Analog Input range

TubularTubular Posts: 4,702
edited 2014-06-23 20:37 in Propeller 2
I've been playing around with a sensor whose output swings down just below zero volts, by up to a diode drop or so

I believe the analog input operation on Prop2 was going to be setup such that 75% of the range covered 0 to 3.3v, so it would be great if the input does indeed go down to -0.8v

Is this still "the plan", or does the synthesis now mean we'll be looking at a more standard range of 0...3.3v (only)

Comments

  • jmgjmg Posts: 15,173
    edited 2014-06-22 22:21
    As far as 800mV would be unlikely, as there will always be parasitic diodes present.
    A couple of hundred mV is all most Silicon devices promise, and then they spec best linearity inside the rails.
  • TubularTubular Posts: 4,702
    edited 2014-06-22 22:32
    Few hundred mV would be fine. Am using 300mV range on the multimeter, it hasn't "clocked it" yet...
  • cgraceycgracey Posts: 14,151
    edited 2014-06-23 17:03
    In delta-sigma ADC mode, a pin's IN bit reads ~12.5% duty at GND and ~87.5% duty at VDD (ideally 3.3V).

    So, 0% duty should indicate around -0.4125V and 100% duty should indicate about 3.7125V.

    There are calibration modes while in ADC mode: if DIR bit = 1, then OUT bit = calibrate low/high. These connect the ADC input to GND or VDD via identical switches and current-limitting resistors that normally connect the ADC input to the pin. So, GND/VDD calibrations should be very accurate.
  • jmgjmg Posts: 15,173
    edited 2014-06-23 17:14
    cgracey wrote: »
    In delta-sigma ADC mode, a pin's IN bit reads ~12.5% duty at GND and ~87.5% duty at VDD (ideally 3.3V).

    So, 0% duty should indicate around -0.4125V and 100% duty should indicate about 3.7125V.

    What is the input impedance of the ADC at 0.0V,3.3V and at -0.4125V, 3.7125V. (at elevated temperatures) ?
    What is the expected tolerance on that ?
  • cgraceycgracey Posts: 14,151
    edited 2014-06-23 17:57
    jmg wrote: »
    What is the input impedance of the ADC at 0.0V,3.3V and at -0.4125V, 3.7125V. (at elevated temperatures) ?
    What is the expected tolerance on that ?

    Impedance is ~5 megohms. Tolerance is probably +/-10%, based on process conditions. Impedance would increase with temperature, too.
  • TubularTubular Posts: 4,702
    edited 2014-06-23 20:37
    Thanks Chip

    I'm guessing the impedance won't be that high near the extremities as the diodes kick in, but thats ok (useful for clamping, even).

    It'll be a bit like the old Intersil CA3140 that still worked down to a bit below the negative rail... neat
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