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ADC for seismic data acquisition system

Ganesh Yadawate
Prodigy 80 points
Other Parts Discussed in Thread: ADS1258, ADS1281, ADS1282, ADS1257, ADS1262, ADS127L01, ADS1278, THS4521

I want to design seismic data acquisition system (Geophysical Exploration Tool). To convert analog data from geophone into digital format I need to choose high resolution ADC(24 bit). I am looking to use texas instruments ads1258. Can anyone please suggest proper options available from TI and pros and cons of my choice. Thanks!!

  • 0
    TI__Mastermind 44640 points

    Hi Ganesh,

    For seismic exploration, the ADS1281 and ADS1282 are the devices that we recommend for that application. Typically, some of the main concerns are the ADC's AC performance and power consumption, and these devices are optimized in those areas. The main difference between these devices is that the ADS1282 also provides an integrated PGA.

    A typical geophone circuit will often look like this:

     

    While the ADS1258 could work, it's better suited for measuring DC signals. You'll notice that the ADS1258 datasheet doesn't provide much in regards to AC performance specifications and the digital filter is a SINC5 filter (low-bandwidth and not a flat passband). Other examples of ADC's used for DC measurement would be the ADS1257 and the ADS1262.

    ADC's aimed towards AC signal measurements will often provide the SNR and THD performance, and will typically offer a wide-bandwidth filter with a flat-passband response, like the ADS1282. Another example of an ADC used for AC measurements is the ADS127L01.

    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 80 points
    Thank you very much Christopher for your valuable recommendation. I will consider ADS1282 in my design. What type of signal conditioning circuit do I need in this to interface sensor to this circuit?
  • 0 in reply to Ganesh Yadawate
    TI__Mastermind 44640 points
    Hi Ganesh,

    In the case of the ADS1282, a low-noise PGA is included in the ADC so this part of the signal conditioning is taken care of for you. In the circuit shown above, you'll notice that the external signal conditioning consists of only a DC bias, low-pass RC anti-aliasing filter, input protection, and an external reference voltage source.

    You'll want to use good quality passive components, for example, C0G type capacitors will have lower temperature and voltage coefficients of capacitance and will not distort your signal as much as other types of dielectrics. However the trade-off is that C0G's offer limited capacitance values (not much larger than 10 nF) and often require larger package sizes to accommodate the same capacitance as other dielectrics.

    Additionally, take care to limit the amount of leakage current that input protection (diodes or other components) may add to the input-signal path, as higher leakage currents will result in gain errors and other non-linear effects (especially at higher temperatures where diode leakage current increase exponentially). I would recommend looking for low-leakage TVS diodes to use for input protection. Also, using capacitors with higher voltage ratings will help to reduce capacitor leakage.

    Best Regards,
    Chris
  • 0 in reply to Christopher Hall
    Prodigy 80 points

    Thank you again Christ for your detailed information. One last question I would like to ask, is there any 8 or more  channel version of ADS1282 available?

  • 0 in reply to Ganesh Yadawate
    TI__Mastermind 44640 points
    Hi Ganesh,

    You're welcome!

    Unfortunately, we do not have a higher channel count version of the ADS1282. Typically, multiple ADS1282's will be used to provide higher channel counts. Note that even though the ADS1282 provides for 2-channels, it's not practical to multiplex between these channels due to the long settling time of the digital filter.

    If you need 8 channels, another alternative to 8x ADS1282's might be the ADS1278, 8-channel simultaneously sampling, 24-bit delta-sigma ADC. You would need to provide the input buffer/gain stage for each channel, something like the THS4521. The ADS1278 would not have quite as good of performance as the ADS1282 and each input would have a fixed gain instead of a programmable gain.

    Best Regards,
    Chris
  • 0 in reply to Christopher Hall
    Prodigy 80 points
    Hi, I have to make this circuit design for minimum 8 channels. What processing do you suggest to multiplex output of 8 IC's?
  • 0 in reply to Ganesh Yadawate
    TI__Mastermind 44640 points

    Hi Ganesh,

    I'm sorry I didn't see your last question sooner...did my last response help?:

    Christopher Hall said:
    If you need 8 channels, another alternative to 8x ADS1282's might be the ADS1278, 8-channel simultaneously sampling, 24-bit delta-sigma ADC. You would need to provide the input buffer/gain stage for each channel, something like the THS4521. The ADS1278 would not have quite as good of performance as the ADS1282 and each input would have a fixed gain instead of a programmable gain.

    If by "processing", you're referring to the microcontroller, I would recommend using something that you're familiar with, if possible. However, it's common to see FPGAs used for this application since there is often a lot data processing involved, but if you intend to use a PC to process the data, then you could get away with using a slower processor. At a minimum, you'll need to be able to provide a fast enough SPI interface to be able to clock out 192 (8x24) bits after every ADC conversion period, and leaving some additional overhead to do some processing or passing the data to memory. 

    Being more familiar with the ADC side of things, I don't really have a specific recommendation for any particular processor here...you're options are pretty wide open!

    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 80 points

    Thank You Chris very much again. I am starting my design with ads1282. But for end product development I may consider using ads1278. I will experiment on both of the ICs. Thanks again!!