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I wonder for the numerical values in table and graph..??(pga280)

Sung ho Cho
Prodigy 190 points
Other Parts Discussed in Thread: PGA280

In bottom graph, we can see the noise voltage at a gain 128 is around 20nV, along from 0.1 to 100k frequency range. But in upper table, we can see the noise voltage is 420 Vpp at f=0.01Hz to 10Hz..

I don't know how I anlayze this matter.... please help me. 

  • 0
    TI__Mastermind 27715 points

    Hello Sung ho Cho,

    You are looking at the same noise performance of the PGA280, but presented in two different ways.

    The line in the table which you've highlighted shows the integrated input voltage noise over a fixed frequency range (0.1Hz to 10Hz), in units of nVpp. 

    In the plot at the bottom, we show the input voltage noise spectral density over frequency, in units of μV/(√Hz). In order to convert voltage noise spectral density to the integrated peak-to-peak noise voltage, you must square the voltage spectral density to convert to power spectral density, integrate over a specific frequency range, take the square root, and finally multiply by 6. 

    Let's do this below, starting with the G=128 broadband noise value of 22nV/(√Hz).

    • (22nV/√Hz)2 = 4.84e-16 V2/Hz
    • 4.84e-16 V2/Hz * 10Hz = 4.84e-16 V2
    • √(4.84e-16 V2) = 6.957e-8 VRMS
    • 6.957e-8 VRMS * 6 = 4.20E-7 VPP = 420nVPP

    As you can see, the math works out properly. For more information on noise analysis, see this presentation by Art Kay: ftp://ftp.ti.com/pub/linear_apps/noise_article_series/article9-noise2-oct-2006.pdf

    Best regards,

    Ian Williams
    Linear Applications Engineer
    Precision Analog - Op Amps

  • 0 in reply to Ian Williams
    Prodigy 190 points

    First of all, I'm really appreciate your reply.

    I have another question... Why does datasheet pick specific frequecy region(0.01Hz to 10Hz)?
    Is there special reason for those frequency region?
    And if I put the voltage source(1Hz) into the pga280 input, which noise value is applied, 420nVpp or 22nV/(√Hz)?
     
    Regards...
  • 0 in reply to Sung ho Cho
    TI__Mastermind 27715 points

    The region of 0.1Hz to 10Hz noise is a fairly  industry-standard representation of low-frequency noise. For example, 0.1Hz corresponds to 10 seconds, so that plot shows what kind of noise to expect over a 10-second time interval - as long as you filter out the noise content above 10Hz. Going all the way down to 0Hz is not practical, since 0Hz corresponds to infinite time.

    If you apply a 1Hz signal, you will see a noise value that corresponds to the PGA gain you have selected and the frequency range allowed by whatever filter is present on your circuit. To predict this, you can look at the broadband noise values given in the noise spectral density plot and follow the same steps in my last message to convert to peak-to-peak noise.

    Best regards,

    Ian Williams

  • 0 in reply to Ian Williams
    Prodigy 190 points

    Thank you very much Ian... Have a nice day..~!!