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Basic questions of ADS8422

Takumi Suzuki1
Mastermind 8430 points
Other Parts Discussed in Thread: ADS8422

Hi,

I woule like to ask you some basic questions about ADS8422IB as below.

Would you please teach me?

1) No missing codes

About ADS8422IB, datasheet describes that the missing code is 16bits(MIN).

This means "there are not missing codes", I think.

But the DLE of ADS8422IB is specified -1(MIN), +/-0.7(TYP), 1.5(MAX) on the datasheet.

From this DLE value, I suppose the missing codes of ADS8422I is 15, not 16...

What is misunderstanding of my idea?

2) Resolution

About Resolution, as ILE is specified -2(MIN), +/-1(TYP), 2(MAX) on the datasheet, I think that the resolution of ADS8422IB is 14bit actulally.   

(About ADS8422I, as the ILE is -6(MIN), +/-2(TYP), 6(MAX), the resolution is actually 10bit !?...)

Is my idea correct?

3) Input 

What is "Fully Differential Input with Pseudo-Bipolar" ?

Thank you for your kindness.

  • 0
    TI__Expert 8975 points
    Hello Takumi-san,

    Please find my comments below.

    1. No missing codes - The ADS8422 device does not have any missing codes. The DLE = -1 (MIN) specification comes due to rounding off of the actual MIN value. We have since then shifted away from the rounding off & datasheets of the new parts will not show DNL = -1.
    I can assure that the device does not have any missing code.

    2. Resolution - The INL specification lets you know the actual transfer characteristic of the ADC against the ideal transfer characteristic of the ADC. The difference between ideal & actual curve is expressed as INL. This should not be referred to as resolution of the device.
    The effective resolution can be derived from the SNR or SINAD specification by using the formula
    ENOB = (SINAD - 1.76)/6.02

    You can refer below blogs/app notes which discuss these parameters.
    ADC-accuracy Part1:- e2e.ti.com/.../adc-accuracy-part-1-is-accuracy-different-from-resolution
    ADC-accuracy Part2:- e2e.ti.com/.../adc-accuracy-part-2-total-unadjusted-error-explained
    Application Note:- www.ti.com/.../slaa587.pdf

    3. Input -
    The device is capable of converting a differential input signal within -VREF to VREF range. Each input of the differential signal is limited to swing between GND to VREF (typically) or -0.2V to VREF + 0.2V (absolute range).
    Since none of the inputs is not allowed to swing bipolar (+ve & -ve voltage up to Vref) bur still differentially we convert bipolar voltages, this combination is referred to as "Fully differential input with Psuedo-bipolar".

    I would like to know more about the application you are targeting with ADS8422 devices. It will be great if you can share some details regarding the same.

    Thanks & Regards,
    Shridhar.
  • 0 in reply to Shridhar%20More
    Mastermind 8430 points
    Hello Shridhar-san,

    Thank you very much for your detail explanation.
    I think I could understand but need to study more by the documents you told me.

    About the ADS8422, sorry, I do not have the real application.
    But ADS8422 is a typical ADC of TI and it is introduced on the documents TIJapan made.
    So I have studied ADS8422 to understand ADC.

    Again, thanks for your kindness.