powering a 16 bit ADC

Discussion in 'General Electronics Chat' started by Gibson486, Feb 10, 2014.

  1. Gibson486

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    Jul 20, 2012
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    I am trying to power a 16 bit ADC. Do I need to pick a really low noise regulator (or even a precision reference)? I am choosing between the ADS1118 and the LTC2470. It will be used in conjunction with a Honeywell HSC series pressure transducer and a low noise op amp (to bring it down to the correct input voltage levels).
     
  2. t06afre

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  3. Gibson486

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    need 16 bits....
     
  4. tshuck

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    16 bits with all but the higher reference voltages will require very careful board layout and, probably, shielding.

    Perhaps if you explained why you need the 16 bit resolution and what the goal is, we might be able to offer some alternatives - that is, assuming this is a hobby/non-professional project.
     
  5. t06afre

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    Do you have any datasheet for your transducer? Also perhaps you can post a drawing on what you plan to do:)
     
  6. crutschow

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    If the ADC's have a built-in reference voltage than you shouldn't need an external one.
     
  7. Gibson486

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    I use the HSC 150 psi transducer with analog out put. I need .01 psi accuracy. I could use a 14 bit one, but 14 bit cost more than a 16 bit one. 12 bit is not accurate enough.

    The reference from the LTC part is only 1.25V.
     
  8. tshuck

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    All of the sensors I'm seeing from Honeywell have either a SPI or I2C interface. Why aren't you using those?
     
  9. Gibson486

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  10. MrChips

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    Don't believe that by going to a 16-bit ADC you will get 16-bit resolution.

    Firstly, you will have to amplify your low level signal to bring it up to the range of the ADC.

    Secondly, you have to design all your analog supplies and references properly to minimize supply noise.

    Thirdly, you have to ensure that the analog and digital supplies and grounds are properly designed and filtered to prevent digital noise from corrupting the analog circuitry

    16-bit resolution is asking for 0.0015% accuracy. Be prepared to lose a couple of bits as a result of noise.
     
  11. crutschow

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    You may get 16-bit resolution but you are unlikely to get 16-bit accuracy from a 12-bit transducer. Is it 16-bit accuracy you need or is 16-bit resolution good enough?

    If you just need resolution then you can digitally average the 12-bit digital signal in a uP to improve the resolution. Assuming there is a at least a 1-bit dither in the signal due to noise, then the resolution can be increased by the square-root of the number of samples averaged. Thus to improve the resolution by 2-bits from 12 to 14 bits (a factor of 4) you would do a cumulative running (moving) average of at least 16 samples. Of course you must keep this average value of the signal to at least 14-bits to get the desired resolution.
     
    Last edited: Feb 10, 2014
  12. GopherT

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    These HSC sensors are available in analog -or- 12 bit digital (use 14 bit I2C communication)
    They spec accuracy so nowhere near the OP's design spec of 0.01psi/150 psi which translates to 1 part in 15000 or 0.0066%.

    According to the datasheet...
    Extremely tight accuracy of 0.25% FSS BFSL (Full Scale Span Best Fit Straight Line)
    Total error band of 2% full scale span maximum​
     
  13. Gibson486

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    I only need 14 bit to really get what I need, but 16 bit is cheaper. I am prepared to lose bits.

    I have already designed the "bridge" to bring the signal up to full scale.

    I already know the ground design will be a headache...

    I was thinking that no regulator can give me the regulation I need, so that is why I was asking about using a reference.

    BTW, my bad, I need 100 psi. I was reading from the datasheet when I said 150.
     
  14. Gibson486

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    Hmmm....I will look into this....
     
  15. MrChips

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    Just to recap what has been said before, you need to measure 100psi with 0.01psi resolution. That translates into 1 part in 10000, or 0.01%. You will need 14-bit resolution and hence a 16-bit ADC is an appropriate choice.

    Most ADCs come with internal reference. You do not use a voltage reference to provide power to your ICs.

    Make sure all analog, digital supplies and voltage references are properly decoupled with 0.1μF ceramic caps and 1-10μF electrolytic tantalum caps.

    Your emphasis would be on designing the PCB using recommended power and ground layout practice. Use separate analog and digital supply lines and ground.
    Use ground planes extensively as much as you can. Generally I make the entire bottom layer a ground plane. Where applicable, use separate ground planes for the analog and digital circuitry.

    If the analog and digital circuits are powered from the same supply source I would add ferrite beads to filter the supply lines.
     
  16. vrainom

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  17. ramancini8

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    Assuming a 5 volt power supply, and 16 bits, one bit is approximately 0.76 uV or 7.6 (10E-5) volts. The formula is: one bit = Vref/2E#bits. It takes a real experienced engineer to obtain this accuracy. Amplification/level shifting, wiring noise, power line noise, temperature change, and circuit board layout all contribute errors. If you only need 14 bit accuracy you can use a 16 bit converter and discard the lower bits. I was an applications engineer for Analog Devices for several years, and I watched many good solid engineers go down in flames trying to solve this problem because it was unsolvable.
    Another thing to consider, 12 accurate bits is worth more than 16 inaccurate bits because bit errors cause all kinds of weird problems like control system lockup. I once trouble shot a customer design that malfunctioned because a high current wire was run too close to the converter; yes, it is that picky.
     
  18. Gibson486

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    It is the HSC sensor I am worried about.... It's ratiometric...so noise will simply get reflected onto the output.
     
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