Boosting AD8642 output current

I'm testing a design in which I need to do a current to voltage conversion. The current ranges from picoamperes to 100mA (both positive and negative), so I use 4 different resistors and a switch to select each of them. I filter the corresponding signal (not shown in the image), use a variable gain (x1, x10, x100, x1000) amplifier and then adapt it and detect it. 

Everything works perfectly fine till 20mA, the op amp output current limit. Can anyone please show me a simple power stage that can be used preferibly inside the feedback loop? The output voltage has to be between -5 and +5 volts and has to work for +-100mA.

  • 0
    •  Analog Employees 
    on Jun 26, 2013 12:09 AM

    Hi Damaso,

    Thanks for using ADI products on your application. If replacing the AD8642 with another opamp that has a higher output drive capability is not an option for you then you can try to implement the circuit below.

    There are other power/current booster design circuits on the internet which you can also consider and implement.

    Also, I noticed from your schematic that there is a short between the output pin of U2 and the negative supply. I would recommend that you avoid this in your actual circuit

    I hope this helps!

    Best regards,

    Emman

  • Thank you for the answer. I'll try and implement that circuit and analyze the results. And yes, the output connected to ground was a mistake I made when I modified the schematics to post it here.

    My design uses the other op amp present in the AD8642 to make the same operation with another current source. If i'm not wrong, this power stage won't work in that case. Could you please correct me or show me an alternative? Should I use two AD8641, one for each current conversion, to avoid this problem?

  • The circuit works quite as expected for currents above 1uA. But when I connect the 500Kohms feedback resistor and try to meassure the current, power source shows a current consumption of at least 400mA in both rails. It seems like a start condition problem with that configuration of the feedback loop, because it doesn't happen if I select the 500ohms and change it to the 500Kohms during a meassure.

    I'm thinking that maybe what does work for high currents (mA) won't work or won't meet the expected behaviour for the uA, nA and pA currents. That's why i'll try and use two different op amps for above mA and below mA. I'll keep the AD8642 for the low current meassurement, thanks to its femtoampere bias current, but I'll need a new op amp capable of 100mA output current and +-12V sources. Would anyone please recommend a part for my project?

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:54 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