• AVR Freaks

PIC24FV32KAxxx ADC Nonlinearity.

Author
dkemppai
Starting Member
  • Total Posts : 75
  • Reward points : 0
  • Joined: 2012/04/30 11:23:04
  • Location: 0
  • Status: offline
2012/06/22 12:06:38 (permalink)
0

PIC24FV32KAxxx ADC Nonlinearity.

Hi all,
Doing some testing of an ADC on a PIC24FV32KA304. ADC is set for 12Bit.
Slowly sweeping the input voltage, there appears to be a rather large nonlinearity at 2048 of about 30 counts. A second one at 3072 of about 25 counts and another one around 1536. Not surprisingly these are on a power of two boundry. I was surprised how large they are. Anyone else run into issues with the ADC and nonlinearities?
 
Otherwise, they ADC appears relativley linear. Full scale average linearity looks good.  (If you ignore the noise of between +/- 5 to +/- 10 counts, but that could be my source, or power supply rail).
FYI, source is slow sweep triangle wave (2.5mHz) and through 10Khz LPF opamp driven source. 
 
Dan
 
#1

4 Replies Related Threads

    Mike017
    Super Member
    • Total Posts : 2698
    • Reward points : 0
    • Joined: 2007/08/25 08:11:31
    • Location: Lincoln, RI, USA
    • Status: offline
    Re:PIC24FV32KAxxx ADC Nonlinearity. 2012/06/23 13:41:53 (permalink)
    0
    Hi,
    FYI, source is slow sweep triangle wave (2.5mHz) and through 10Khz LPF opamp driven source.
    --Maybe you already checked but there is an analog voltage impedance specification in the electrical specs.
    --Big difference in the ADC accuracy specs compared to a PIC24FJ128GA010. (attached) [24FV on the left, 24FJ on the right]
     
    Good Luck,
    Mike
    post edited by Mike017 - 2012/06/23 19:02:20

    Attached Image(s)

    #2
    Muenchow
    Super Member
    • Total Posts : 282
    • Reward points : 0
    • Joined: 2012/02/20 02:25:22
    • Location: Bochum Germany
    • Status: offline
    Re:PIC24FV32KAxxx ADC Nonlinearity. 2012/06/23 16:22:47 (permalink)
    0
    Did you notice the funny "copy and paste" error in the specification of 12-Bit ADC. It's the first ADC in the world that "guarantees" monotonicity with > +/-1 LSB differential error. LoL
    I wonder if the numbers in the table have been determined with the same accurateness?
     
    Apart from this curiosity, I agree with the consideration, that the table results most likely depend on prerequisites like source impedance, that may be violated in your test.
    #3
    dkemppai
    Starting Member
    • Total Posts : 75
    • Reward points : 0
    • Joined: 2012/04/30 11:23:04
    • Location: 0
    • Status: offline
    Re:PIC24FV32KAxxx ADC Nonlinearity. 2012/06/26 07:42:35 (permalink)
    0
    Mike017

    Hi,
    FYI, source is slow sweep triangle wave (2.5mHz) and through 10Khz LPF opamp driven source.
    --Maybe you already checked but there is an analog voltage impedance specification in the electrical specs.
    --Big difference in the ADC accuracy specs compared to a PIC24FJ128GA010. (attached) [24FV on the left, 24FJ on the right]

    Good Luck,
    Mike

    I have a 1K resistor in series with the Active output of the LPF, as the datasheet suggests. However I did just notice that the values in the datasheet are based on an external +/- VRef. I'm using the supply rails, which may be getting me into some trouble. My rails are relatively quiet, however not noiseless. That being said, I would expect the reference noise would appear to be evenly distributed, and not appear to be larger at certain bit value boundaries. (Such as 2048, etc).
     

    As far as the comparison, does this mean that the ADC's are very comparable? The 12bit unit has an LSB that is effectively 4 times smaller than the LSB of the 10bit ADC, in terms of converted voltage. If you look at nonlinearly and error in terms of full scale input voltage, are they not similar? (Logically, this makes sense, however I'm 100% certian of this).  If they are similar, what's the point of the extra 2 bits of data?
     

    Maybe I'll run som testing in 10bit mode, to see how it compares.
     
    Dan
    #4
    Mike017
    Super Member
    • Total Posts : 2698
    • Reward points : 0
    • Joined: 2007/08/25 08:11:31
    • Location: Lincoln, RI, USA
    • Status: offline
    Re:PIC24FV32KAxxx ADC Nonlinearity. 2012/06/26 17:08:10 (permalink)
    0
    Hi,
     
    Really good thread on ADC considerations  (see AN688) that came from the Hardware Gallery. If you post your schematic something may show that is worth noting i.e. proper caps, separate Avdd/AVss routing, etc. Also, it's worth rechecking with an external +/- Vref.
     
    Good Luck,
    Mike
     
     
     
     
    post edited by Mike017 - 2012/06/26 17:19:40
    #5
    Jump to:
    © 2019 APG vNext Commercial Version 4.5