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ADS131E0x dc coupling

Oleksiy Pazyuk
Expert 2585 points
Other Parts Discussed in Thread: ADS131E08

Hi,

Could someone give comments  for the next question.

In the first  page of the datasheet for ADS131E08 is mention next:

  • Supply range:
    • Analog:
      • +3V to+5V(Unipolar)
      • ±2.5V (Bipolar, allows dc coupling)

As I understand the phrase "Bipolar, allows dc coupling" is related to the direct coupling for the current sensing, because for the  voltage sensing we could implement dc coupling regardless any kind of the analog supply.

May be  I miss something out and with unipolar analog supply is not possible to make voltage sensing with dc coupling?

Could someone comment or give practical based expertise?

  • 0
    TI__Mastermind 23235 points

    Mr Pazyuk -

    The text references the fact that with unipolar (AVSS = GND or ~0V), you cannot DC couple since the inputs cannot go below ground.  Using split supply, the possibility exists for the input to go below ground.  Since we are measuring voltage for power monitoring, their is usually minimal offset (50/60 Hz is centered at ~0V).

    So you can do unipolar measurements as long as the DC value is within the common mode and input range of the ADC.

  • 0 in reply to Greg Hupp
    Expert 2585 points

    Hi Greg,

    Greg Hupp said:
    The text references the fact that with unipolar (AVSS = GND or ~0V), you cannot DC couple since the inputs cannot go below ground. 

    Ac I understand for voltage sensing we use dc coupling anуway, thus  when  we use common voltage shift as it mention in the datasheet (see picture below) there is no difference either we have unipolar supply, either bipolar. Am I correct?

  • 0 in reply to Oleksiy Pazyuk
    TI__Mastermind 23235 points

    Mr Pazyuk -

    As mentioned, if you level shift the inputs to within the common mode range spec, then using a unipolar supply should not be a problem.  The above circuit provides the DC bias that would be required to used the device in unipolar mode.

  • 0 in reply to Greg Hupp
    Prodigy 65 points

    Hi Greg,

    Similar questions...
    1) You said unipolar supply should not be a problem as long as the input shifts are in the common mode range (input span), which may mean the unipolar case also can measure dc offset (so dc coupling). Then, why does it mention only bipolar supply allow dc coupling?
    2) Can I understand that dc coupling is similar with dc offset measurement?

  • 0 in reply to Hongki Jo1
    TI__Mastermind 23235 points
    Hello Hongki -
    When operating the device with uni-polar supplies (eg AVDD = 5V and AVSS = GND), the input signals should meet the input common-mode range requirements shown on page 41 of the datasheet. If they do this, you can measure DC values.
    For some measurements, the input signal is referred to ground, which can violate the common mode range when operating with +5V and ground as the supplies. Generally, you will need to use the +/-2.5V supplies for measuring input signals that are ground reference to ensure that you meet the common-mode range requirements.