ad420 DC level



I need to drive a laser at ~12-14 mA DC, the laser has an impedance of ~163 ohms. The maximum allowed drift is +/-0.6 mA.


We have been looking at ad420, but I have some questions to insure the it is suitable for our application.


  • Is the output impedance of it is suitable for my use?
  • If not can you recommend an op amp that can lower the output impedance?
  • Is it able to hold a constant DC level over long over a long period of time


Hope you can help

  • 0
    •  Analog Employees 
    on Nov 15, 2015 11:39 PM


    To answer your questions, the output impedance of the AD420 in current mode is 25MΩ which is suitable when you compare the 163Ω impedance of your LASER. The part has a maximum offset drift of 50ppm/°C and a maximum total output error drift of 50ppm/°C.

    If you are concerned about the drift, you may want to use the external boost function of the part as it can reduce the amount of temperature induced drift in the part. This will minimize the temperature induced drift of the on-chip voltage reference, which improves drift and linearity.

    Thank you for considering the AD420. ADI is offering an alternative part which is the AD5420. The AD5420 has higher output impedance in current mode (50MΩ). More details can be seen in the data sheet.



  • Hi,

    As i understand it then the source impedance should be lower then the load impedance is this not correct?



  • 0
    •  Analog Employees 
    on Nov 16, 2015 5:04 PM

    Hi Morten,

    If the signals you are interested in are voltages then you are correct that you would need low impedance at the output. The reason behind this is we want to the avoid the loading effect that the output may produce should it have higher impedance than that of the load.

    However, the opposite is true when the signals that you are interested in are current. Having a significantly higher output impedance would mean that the more current will go to the load instead. This minimizes the loading effect on the output.

    I assumed that you are interested in current because the original post is asking about the driving a certain amount of current to a LASER device. Please do correct me if I am mistaken.

    I hope this helps. If you have any other questions, please do not hesitate to ask.



  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:16 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin