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ADS1282: REF about ADS1282

Ruiqi Huang
Prodigy 30 points

Part Number: ADS1282

Accordding to the datasheet , ADS1282 has a good SNR which is 130db,it means that  Vs/Vn=10^6.5.  when I want to measure a signal of 1V, the noise  should be less than 1V/(10^6.5)=0.4uV. but the Vref we used in the ADC is REF5025 ,which  has  3uV noise, so it can't reach the best quality. which VREFcan we used to reach a better peformance?it must has less than 0.4uV. Thank you.

  • 0
    TI__Mastermind 44780 points

    Hi Ruiqi,

    Welcome to the TI E2E Forums!

    On it's own, I don't believe you will find a reference with low enough noise performance to match the ADS1282. Almost any reference source will require some external circuitry, typically in the form of a large RC filter, to reduce the reference noise.

    Without any external filtering, the ADS1282's digital filter will help a bit in removing some of the reference noise. At a data rate of 250 SPS, the FIR filter's noise bandwidth is around 100 Hz; however, this is still larger than the 10 Hz noise bandwidth that the 3 uVpp/V specification uses, so a single pole RC filter would need to have a cutoff frequency of less than ~10/1.57 = 6.37 Hz (1.57 Hz is the a scaling factor that relates the -3dB cutoff frequency to the effective "brickwall" noise bandwidth of the filter) to reduce the noise any amount beyond what is specified in the REF5025's datasheet.

    Another thing to note is that reference noise will scale with the amplitude of the input signal. Therefore, for small input signals the reference noise might be insignificant. It is only for large input signals that you will notice the effects of reference. Reference:

    e2e.ti.com/.../the-impact-of-voltage-reference-noise-on-delta-sigma-adc-resolution

    I hope that helps!

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 30 points

    Thank you very much for your reply, and I still have  another problem. as the ADS1282 need a voltage supply of -2.5V and +2.5V,and the VREF is also in this range. but it may cause the ADC system in a float ground, because we use a LDO  to offer supply +2.5V and -2.5V. as a result ,the 50HZ supply noise may come into the system. How can I solve this problem?

  • 0 in reply to Ruiqi Huang
    TI__Mastermind 44780 points
    Hi Ruiqi,

    I'm not so sure I understand your question/concern... The +/-2.5V supplies will need to be referenced to same ground plane as the ADC and so the ADC ground won't be floating (with respect to the supply rails).

    The LDOs should also offer you good power supply rejection of 50 Hz noise. With that in mind, if you also have proper supply decoupling on the analog and digital supplies you shouldn't have an issue with 50 Hz noise.

    Best regards,
    Chris
  • 0 in reply to Christopher Hall
    TI__Mastermind 44780 points
    Hi Ruiqi,

    Did I sufficiently answer your questions?

    Thanks and best regards,
    Chris
  • 0 in reply to Christopher Hall
    Prodigy 30 points

    Hi Christopher

    Thank you for you reply, I think I need more experiment to test this . 

    Best regards