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Adc tlc2543 accuracy — Parallax Forums

Adc tlc2543 accuracy

Stamp_1Stamp_1 Posts: 5
edited 2009-09-08 15:50 in BASIC Stamp
I am new to the forum and have been trying to work this problem out but can't seem to.
So any help would be appreciated.

Using BS2 with USB BOE.
TLC2543 with shortest wires possible.

Using Tracy Allen's code for this chip and it does work very well only using one input·channel for now, code has not been altered wanted to make sure it was working correctly before I changed it at all.· Since this project is for·reading a 0 - 5v transducer I thought I would start with variable power supply for an input to the TLC2543 instead of an actual transducer.· Here are my results

Input············· Output (debug screen)
508mV··············· 507mV
1.006V···············1.005V
1.506V···············1.510V
2.0V················· 2.006V
2.5V··················2.507V
3.006V················ 3.017V
3.504V··············· ·3.515V
4.006V··············· ·4.517V
4.998V···············4.998V

Is this typical, don't believe it would be power supply noise / ripple since the lower and upper voltages and good.· Accuracy is very important because the project requires a .2PSI accuracy range.

Thank you

Comments

  • FranklinFranklin Posts: 4,747
    edited 2009-08-29 21:17
    Are the values repeatable or do they fluctuate.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    - Stephen
  • Stamp_1Stamp_1 Posts: 5
    edited 2009-08-30 14:00
    Yes, it does repeat.· Actually it would have been good if it didn't repeat this way I would have something to go on.
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2009-08-30 17:21
    That is more error than I would expect. That next to last value, 4.006 V in measured as 4.517 is so far off there is no question that there must be something wrong. What are you using as a reference for the TLC2543? Also, the power supply that you are using to supply the input, what is that, and are you sure those values are correct?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Tracy Allen
    www.emesystems.com
  • Stamp_1Stamp_1 Posts: 5
    edited 2009-08-31 17:45
    The way I have the TLC2543 hooked up is

    Power for the chip is coming from the VDD & VSS connectors on the BOE
    Reference is connected to VDD & VSS.· (VDD is 4.997V)
    Input to the channel is a variable power supply 0 - 5V simulating a transducer (typical bench supply Protek)
    Verification of input voltage is done by a Fluke DVM.

    I will go back and again verify my voltages.
  • Stamp_1Stamp_1 Posts: 5
    edited 2009-09-07 14:35
    Went back and verified my voltages and they are still the same.

    Input············· Output (debug screen)
    508mV··············· 507mV
    1.006V···············1.005V
    1.506V···············1.510V
    2.0V················· 2.006V
    2.5V··················2.507V
    3.006V················ 3.017V
    3.504V··············· ·3.515V
    4.006V··············· ·4.517V
    4.998V···············4.998V

    Is it possible that my fairly typical breadboard layout could have crosstalk with wires next to and crossing over each other?
  • stamptrolstamptrol Posts: 1,731
    edited 2009-09-07 21:10
    If you are using Vdd as the reference, there's a good chance its moving around a bit. Can you redo your test but also include Vref?

    You mention short leads on the chip. Have you also connected a bypass cap to the power supply pins?

    While it shouldn't matter, are the unused channels pulled low?

    Cheers,

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Tom Sisk

    http://www.siskconsult.com
    ·
  • Stamp_1Stamp_1 Posts: 5
    edited 2009-09-08 00:13
    Thank you Stamprol didn't think about Vdd moving around, and monitoring Vref would be a good idea which I will do.
    After that I will put in a ·bypass cap to see if that also helps.
    And all the other channels are floating.

    Just so I know what is causing this issue it will be done in steps instead of all at once.

    ·
  • Greg LaPollaGreg LaPolla Posts: 323
    edited 2009-09-08 01:49
    I use the TLV2543 (which is the same but a 3.3 volt version) on the propeller, I had some deviance, not as much as you are having. I put a .33u Tantalum cap across ref+ and ref- that seemed to help. If you are using a 9V battery to power the BOE, you might try a fresh battery.


    Greg
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2009-09-08 15:50
    It is suspicious to my way of thinking that the results from tests 9 days apart were identical, down to the millivolt, on both the input and the output.

    from 8/29:
    ...
    3.006V 3.017V
    3.504V 3.515V
    4.006V 4.517V
    4.998V 4.998V

    from 9/7:
    ...
    3.006V 3.017V
    3.504V 3.515V
    4.006V 4.517V
    4.998V 4.998V

    Huh?

    In any case, it's hard to explain the TLC2543 reading being 0.5 volt high at 4.006 volt input. I'd take a few more readings between 3 and 5 volts in order to zoom in on the error. If it is a circuit problem, I'd agree with Tom and start by looking at the power supply. Maybe there are serious spikes in supply or reference. However, noise on the supplies usually appears as inconsistent readings.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Tracy Allen
    www.emesystems.com
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