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ADS117L11: Unipolar input mode and negative inputs

Pascal Regache
Prodigy 130 points
Part Number: ADS117L11
Other Parts Discussed in Thread: INA851

Tool/software:

Hello, I am using the ADS117L11 in a unipolar differential input mode as described in section 8.3.1 of the data sheet.

I have AVDD1 = 5V and AVSS = GND. A front end INA851 does my unipolar to differential conversion with the ADC VCM output setting the common mode.

So I have expecting that the ADC would convert (AINP - AINN) into a signed 16-bit value with -Vref < AINP - AINN < Vref.

Then reading section 8.5.7.1, table 8-15 seems to imply that only the 0 to Vref input range is accepted in the unipolar input range ("If the input is configured for unipolar mode, the conversion data are coded in straight-binary format to represent only positive signals over the 0 to Vref").

I do not get it. What is the point of having a differential input (shitted in common mode) if AINP - AINN MUST be positive ? What am I missing here ?

How should I configured the ADS17L11 to fully benefit from a differential signal witth common mode ? 

Thanks

Pascal

  • 0
    Prodigy 130 points

    Hello,

    Could it simply be that "the unipolar input mode" in section 8.3.1 has nothing to do with the one in section 8.5.7.1 and the later is simply related to the register CONFIG4 bit BIP_UNI. And I am the only one confused :-) ?

  • 0 in reply to Pascal Regache
    TI__Mastermind 45645 points

    Hi Pascal,

    The primary support engineer is out this week, please expect a delayed response or clarification. Thanks for your patience.

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 130 points

    Hello Dale, is the primary support engineer back ?

    Best regards
    Pascal

  • +1 in reply to Pascal Regache
    TI__Mastermind 46490 points

    Hello Pascal,

    Sorry for the late reply.  Yes, your comment that "the unipolar input mode" in section 8.3.1 has nothing to do with the one in section 8.5.7.1 and the later is simply related to the register CONFIG4 bit BIP_UNI is correct.

    I can see how the terminology is a bit confusing.  In section 8.3.1, Unipolar and Bipolar refer to the actual supply voltage configuration of the device.  In unipolar, AVSS=GND and AVDD1 is set to a single positive supply voltage.  Assuming AVDD1=5V, each of the input voltages must be in the range of 0V to 5V.  A negative differential voltage is obtained by setting the voltage on the AINN input greater (but still a positive voltage relative to AVSS=GND) than the AINP voltage.

    In section 8.5.7.1, the terms bipolar and unipolar have nothing to do with the actual supply voltages and input voltages; only to the coding for the conversion result.  Even though you have the supply voltages configured for unipolar operation (AVDD1=+5V, AVSS=GND), you will want to set the conversion data format to bipolar input mode (BIP_UNI bit=0b in the CONFIG4 register, which is the power-up default setting).

    I hope this clears things up!

    Regards,
    Keith Nicholas
    Precision ADC Applications