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16-ch, 24bit AIO

Jayakrishnan Vamadevan
Prodigy 110 points
Other Parts Discussed in Thread: ADS1278, ADS1258, ADS1675, ADS1672

Hi 

I need to make a solution for 16 channel differential AIO ( requires ADC and DAC) , with sampling requirements 200ksps . What is the best solution for this requirement

thanks

Jayakrishnan.V

  • 0
    TI__Genius 10615 points

    Hi Jayakrishnan,

    I am from the Precision DAC applications team. I think we will need some more information.

    Do you require high DC precision or are you planning on generating fast waveforms?

    What type of accuracy do you need from the DAC? (+/- 2 LSB)?

    Any extra information about the output specification requirements would be useful.

  • 0
    TI__Guru* 97585 points
    Jayakrishnan,


    I'd like to also find out more about the ADC that you need. Do you really need 24 bit resolution, and do you need that at 200kSPS? If you can live with a lower data rate maybe two ADS1278's may be appropriate.

    The more you tell us about the application, the more we'll be able to help.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 110 points
    Thanks Joseph Wu,

    Yes, my customer need 24bit resolution. On the sampling they said they I need 200kSPS, but I am trying to convince them with a lower kSPS.
    In the ADS1278, does all the 8 channels give 144kHz rate or it is total -which means indvidual channel can give only (144/8) khz ?

    I would like to avoid using 16 ADCs, I would like to go one with multiple channels or get a faster ADC, and then use multiplexers. I hope that is feasible. If so, even with 24 bit , 200kSPS ?
  • 0 in reply to Eugenio Mejia
    Prodigy 110 points
    HI Eugenio Mejia, I think I need sometime to get these information. I am asking my customer to give right info.
  • 0 in reply to Jayakrishnan Vamadevan
    TI__Guru* 97585 points
    Jayakrishnan,


    The ADS1278 would give data at 144kSPS for each channel. There are 8 DOUTs, one for each ADC.

    If they are willing to live with a much lower data rate, the ADS1258 has a 16 channel mux, and can scan through them, single ended, at 23.7kSPS but I believe that's combined for all channels.

    www.ti.com/.../ads1258.pdf


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 110 points

    hi Joseph Wu,

    I have more details about the spec. My application is a PLC ( industrial environment, multi Analog and Digital IO), so ADC should be suitable for that. I have to design all systems around that.

    Here is the spec available as of now.

    24bit preferable , but if that is not possible 16bit ok

    16 diff channels, each should 200KSPS min.

    all analog input is differential  with input range +/-10V

    each channel should be 200KSPS .

    i need total 16 channels, ideally I would like to use two ADC with 8ch each, but worst case I will go forso if the ADC has at least 4 diff input that is great. 

    One qn: If a Multiplexed 2 channel ADC specified with a speed of 500KSPS mean each channel gives 500KSPS or each channel gives 250KSPS each ?

    thanks 

    Jayakrishnan

  • 0 in reply to Jayakrishnan Vamadevan
    TI__Guru* 97585 points
    Jayakrishnan,


    At 24 bits, there are only two devices that go faster than 200kSPS that I know of. The ADS1672 and and ADS1675 could work, but they are both single channel and rather expensive.

    At 16-18 bits, you could go with a SAR-type ADC, but I have more experience with the delta-sigma variety. I'll ask them to respond and see if they have a suggestion.

    A +/-10 V input may also be a bit of an issue. Most of the data converters I know of are +5V (or +/-2.5V) supplies and could only make measurements within the supply range. It's possible that a divide by 2 buffer will be needed.

    A multiplexed 2 channel ADC with a speed of 500kSPS might give 250kSPS for each. It depends on the ADC. There may be setup issues and latency that could slow it down. This would be device specific, and wouldn't be the same for all devices.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 110 points
    Hi Joseph Wu

    In this reply you have mentioned ADS1258 has 23.7ksps, whereas its product page list it as 125ksps. What is the calculation behind this ?

    thanks
    Jayakrishnan. V
  • 0 in reply to Jayakrishnan Vamadevan
    TI__Guru* 97585 points
    Jayakrishnan,


    The 125ksps is for a fixed-channel reading of the device. That would be the data rate for reading for a single channel constantly being used.

    In auto-scan mode, the data rate drops to 23.7ksps. Since the ADC is converting different channels and there is a sinc5 filter, the device automatically goes through 5 samples to ensure the ADC reading is fully settled. There is likely some overhead in sampling different channels which accounts for the extra time to drop the data rate below 25ksps.


    Joseph Wu