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XTR111 input ripple

Vern Shrauger
Prodigy 10 points

Other Parts Discussed in Thread: XTR111

Hello,


We are designing an XTR111 into a sensor circuit that provides a 0-20 mA output.  The external connection to our circuit is 24V, AC or DC.  To keep the cost low, the power supply will be half-wave rectified and filtered with a single capacitor.  This capacitor is sized by current flow in the half-wave rectifier and its ripple voltage.  What level of ripple voltage can the XTR111 tolerate without being reflected in the current output driven?  If a maximum load resistance of 600-ohms is considered, this results in a 12V drop at the load plus 0.6V across the 15 ohm resistors in the datasheet circuit.  This means Vvsp can not drop below 12.6V + some margin to IS before the circuit will enter compliance.  We would like to set this filter capacitor so we do not affect the 0-20 mA performance over a standard 4-20mA load range, maximum 600 ohm.  We want to make sure we are not missing something if we were to set ripple to 5V, for example.


Thanks,

Vern

  • 0
    TI__Guru 64485 points

    Hello Vern,

    There are two parts to the answer to your question.

    First, the XTR111 will not function if the voltage applied to the VSP pin drops below the minimum power supply voltage of +8V.  The compliance voltage of the IS pin relative to the VSP pin is shown in Figures 17 and 18 in the XTR111 product datasheet.  The voltage of the wiring and load should not cause the IS pin voltage to exceed the limits shown in Figures 17 and 18 or the output will not be able to properly regulate the output current.

    The second part is to understand how much ripple will appear on the current output of the XTR111 based on the power supply ripple which can be determined based on the PSRR specification of the XTR111 in Figure 4.  The PSRR figure shows that higher frequency power supply ripples are not rejected as well as low frequency ripples.  The specification is used to calculate the expected change in input offset voltage versus the power supply ripple.  The change in input offset voltage can then be multiplied by the circuit transfer function to understand the effects on the current output. 

    First indentify the frequency of your power supply ripple and the corresponding PSRR specfication at that frequency.  Then you can calculate the expected change in input offset voltage based on the amplitude of the power supply ripple using the following equation:

    Vos_PSRR = Ripple(V) * (1/ (10^(PSRR/20)))

    The output ripple will then be:

    Iout_Ripple = 10*(Vos_PSRR / RSET)