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ADS1282-SP: Input bias current and input impedance

Jeremy Chambon
Prodigy 140 points
Part Number: ADS1282-SP
Other Parts Discussed in Thread: ADS1283

Hello,

I am acutally doing the worst case to analyze what will be the DC offset due to the input bias current from the ADC.

The input bias current in the datasheet which is 1nA represent the differential or absolute leakage current?

 

“The absolute value (IABSOLUTE) is the input current measured in any of the analog input pins.”

“The differential input current (IDIFFERENTIAL) is the current difference measured between the analog input pin pair. This only applies to differential input ADCs”

 

What is the difference between the common mode and the differential input impedance? Is-it possible to have an electrical representation of these impedances?
 

Can i represent the input of the ADC with the follow electric circuit?

 

Thank you very much,

  • 0 in reply to Jeremy Chambon
    TI__Mastermind 44550 points

    Hi Jeremy,

    I believe the input bias current specification is measuring a typical absolute input current. However, depending on the input signal amplitude and ambient temperature, the absolute and differential input bias currents may be on the order of 1 nA...

    The ADS1283 is is a similar device with a similar PGA design, the same input impedance specifications, and its datasheets shows the following typical input bias current plots:

    Regarding the electrical model for the differential and common-mode input impedance, I would draw the circuit as follows (inputs are shorted for measuring the CM impedance):

    I hope that helps!

  • 0 in reply to Christopher Hall
    Prodigy 140 points

    Hello Chritopher,

    Thank you for your reply.

    When the Analog mux is closed inside the ADC, the input bias current and the Rdson of the Analog Mux could create a DC error the mesurement ? Or this DC error is canceled because this error is present on the AINP and AINN signals (if we assume the Ron (30Ohms) and the input bias current are the same on AINP and AINN?

    Does R_DIFF represent the input impedance of the PGA?

    Do you know if TI has some AN or Technical notes about how to calcute an error budget by using an ADC?

    Thank you,

     

  • 0 in reply to Jeremy Chambon
    TI__Mastermind 44550 points

    Hi Jeremy,

    The voltage drop across the MUX's Rdson resistance will cause a small offset error due to the differential input bias current (common-mode IR voltage drops will cancel in the differential measurement). Since the differential input bias current is somewhat proportional to the input signal amplitude, this error may appear as a small gain error which can be reduced by calibration.

    R_DIFF is a first-order approximation of the ADC's differential input impedance, but yes, primarily due to the PGA.

    We don't have a comprehensive app note that explains calculating an error budget. Probably the closest resources to that would be the following...

    TI Precision Labs - ADCs - Error Sources 
    ADC Accuracy Part 2: Total unadjusted error explained