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ADS124S08: References and Input Current

Darren (FAE)
Expert 5483 points

Part Number: ADS124S08

Just want to check understanding of the REFPx and REFNx for a customer.

The datasheet Electrical Characteristics has a section for "VOLTAGE REFERENCE INPUTS" and the REFP1 and REFN1 currents are specified for the input buffers disabled/enabled. Are these values the same for REFP0 and REFN0?

Section 9.3.3.2 and 9.3.3.3 say:

Without buffering, the reference input impedance is approximately 250 kΩ. The reference input current can lead to possible errors from either high reference source impedance or through reference input filtering. To reduce the input current, use either internal or external reference buffers. In most applications external reference buffering is not necessary.

The device has two individually selectable reference input buffers to lower the reference input current. Use the REFP_BUF and REFN_BUF bits in the reference control register (05h) to enable or disable the positive and negative reference buffers respectively.

===========================

So according to this 250kΩ impedance, it makes sense the internal 2.5V Vref would be around 200µA [2 * (2.5V / 250kΩ)].

But REFP1 and REFN1 say ±6µA/V...? Could I get a little bit more clarification?

I really appreciate the help!

Regards,

Darren

  • 0
    TI__Guru** 113765 points

    Hi Darren,

    When using the 2.5V internal reference, the output is already buffered, so in this case it is recommended to turn off the reference buffers.  For the input impedance, this is not a resistance, but is calculated based on movement of charge over time.  The movement of charge in a non-converting condition would be close to zero.  But when the reference is sampled charge is moved from the reference source to the reference sampling caps.  The rate at which the charge moves is relative to the reference voltage applied, the value of the input sampling caps, and the modulator rate at which the reference is sampled.

    Using the impedance value as a calculation for current is really just a rough approximation of the dynamic condition.  As the distribution of charge changes with applied voltage, the electrical characteristics table for the ADS124S08 helps with the approximation by adjusting the impedance value to uA/V.  So the typical response would be 4uA for a 1V reference.  For a 2.5V reference it would be 10uA (4uA * 2.5V).  In your question you are using the maximum current values of 6uA/V, which equates to 15uA for a 2.5V reference and unbuffered input. 

    If you were to use the 250k Ohm impedance number, you would see 2.5V/250k Ohms which equals 10uA.  This is the same result I just calculated using the typical number from the electrical characteristics table.  The number you calculated differed in a couple of ways.  First you doubled the value of the reference voltage.  The full-scale range is +/- VREF/GAIN, but the reference voltage value is just VREF and not 2*VREF.  The second thing is I think you missed a decimal  position in the calculation.  You should have calculated 20uA and not 200uA.

    REF0 and REF1 should have very similar characteristics with respect to the current.  Between the two reference inputs REF1 would be the worst case as the voltage will be switched through both the analog input mux as well as the reference mux.  There could be some potential leakage through the mux over temperature, so REF1 is used as it would be worst case.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Expert 5483 points

    Absolutely beautiful explanation...Thanks :)

    -Darren