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Applying voltage to INPT terminal of ADL5303


We are considering a circuit configration as shown in the figure below.

When switching between passing the current mirror (ADL5315) and not passing the current mirror with the analog switch (ADG783),
a voltage (= ADG783's on-resistance x IPD) is applied at the INPT pin of ADL5303.
Does this voltage affect the accuracy of IPD monitor on the ADL5303?



  • Greetings Hase,

    ADL5303 input voltage is determined by the voltage on it's VSUM pin. Looking at the ADL5303 block diagram (datasheet page 1), we see the NPN logging transistor is driven such that the voltages on the two op-amp inputs are held equal.

    Applied to this application, it means the voltage at INPT holds relatively constant, while the voltage on the other side of the ADG783 switches is what should go up according to IR drop. The DAC output may need higher output voltage setting for higher Ipd current.

    -Bruce H.

  • Hi, Bruce,
    Thank you for your answer.
    I understood that 3 points below.

    1.Voltage on the INPUT pin of the ADL5303 is controlled to be equal to the voltage on the VSUM pin and don't affect the accuracy of current monitor on the ADL5303.
    2.Therefore, in the case of the figure I showed, the voltage after the IR drop of the switch is equal to the voltage on the VSUM pin.
    3.DAC output voltage needs to be increased by the amount of IR Drop to maintain the bias voltage required for photodiodes.


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