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ADS1257: Basic questions about specifications

Takumi Suzuki1
Mastermind 8490 points
Part Number: ADS1257

Hi,

Let me ask you some basic questions about ADS1257 as below.

Q1) INL specifications

Following is a part of System performace table (data sheet page 7)

Q1-1) About INL, the value is only "+", there is no "-" value, +3ppm(typ)/+10ppm(max).

I suppose that it should be described as "+/-" value, that is to say, +/-3ppm(typ), +/-10ppm(max).

What do you think of my idea?

If INL has only "+" value, it seems to be strange that "Gain error" has "+/-" value, +/-0.005%. 

Q1-2) About INL value, the specification is defined "after calibration(Gain/offset, self)", correct?

And is this value from "END-Point" method, right?

Q1-3) INL curve

Following is the INL data, Fig19/page 12.

I have a question about the "shape" of INL curve.

The Shape seems to show around "inverse S" curve, not random.

Will you tell me the reason why INL shows such kind of shape like "S"?

I suppose that not only ADS1257 but also other delta-sigma A/D has similat curve...it's my feeling, sorry.

Q2) ENOB, NFB

You use "RMS" noise and "peak to peak" noise to calculate ENOB and NFB.

About the noise, how did you get them?

Did you measure them by like "bench test" or they are from "simulation" value?

I appreciate your kind support !!

Best Regards,

  • 0
    TI__Mastermind 44620 points

    Hi Takumi-san,

    I will try to provide brief explanations to your questions below:

    INL

    Q1-1) Correct, the INL error is an absolute error so it may be +/- 3ppm, which matches well with Figure 19.

    Q1-2) Yes...offset and gain calibration are typically performed first and then a straight line is drawn between the endpoints of the transfer function. The maximum deviation of the transfer function from this straight line is the INL error.

    Q1-3) This shape is partly due to the end-point INL calculation method but also because most CMOS switches are implemented using a parallel combination of NMOS and PMOS transistors. As the input voltage is swept, the signal will pass through one or both of these transistors and you see a symmetrical "S" shape for the INL error that depends on the input voltage.

    Noise

    Q2) Noise is measured on the bench. Either a fixed number of samples are collected or data is collected over a short fixed-time (10s or less). From the data, the RMS (standard deviation) or peak-to-peak (max - min) codes are computed and used in Equation 1 to calculate ENOB or NFB.

    I hope my explanations are clear. Please let me know if you have any additional questions!

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Mastermind 8490 points

    Hi Chris-san,

    Thank you so much for your reply.

    I understood well!.

    Best Regards,