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How to determaine the output impedance?


I have a VCCS based on AD8130.  After many googling and studying.. now I'm frustrated!

for checking the output capacitance usually I put a 1Kohm resistor on the output and check the 3-dB corner frequency. so the output capacitance is C=1/(2*pi*R*f3dB)

But I really I don't know how I should calculate the output impedance of the VCCS.


  • Hi Ali,

    Few questions.

    1. What made you decide to use AD8130, a differential amplifier? You can use an operational amplifier instead.

    2. What is your current requirement? How much current do you need your amplifier to drive? 100mA, 300 mA, 1 A?

    3. What frequency do you require to operate your VCCS?

    4. Available supply, or required supply you want?

    5. What other critical parameters we need to consider in your application?

    6. What application do you intend to implement?



  • Dear

    the CMRR of the below circuit is mentioned in MT-068 application note:

    I think we can use the above theory in the voltage to current converter:

    but usually there is some mismatch between the resistors so we can add a unity gain buffer like below to improve the CMRR:

    my question:

    what happen to the CMRR if I want to modify the above circuit like below:

    anyhow we have an unity gain at the Vx point but the output impedance seen by this point is 1k||1K=500

    what should I do if I don't want to remove the modified stage? is it nessecery to modify the Rx?


  • Hi Ali,

    Few things.

    1. The CMRR is dependent on how tightly matched the resistors are (RF/RG with RF'/RG'). For discrete components, the highest we can get is 66 dB CMRR for 0.1% tolerance. The external amplifier will have a little or no effect on the CMRR of the difference amplifier as we speak.

    2. There are components, Difference Amplifiers, that offer higher CMRR but I don't think it's best suited for your application and a lot of factors are to be considered.

    3. There is, however, a best viable solution for you, which is a Howland Current Source. I could help you design one if you could provide me the information I asked on my previous reply. The output impedance of HCS will be dependent only on the impedance you hook on the output. We also need to know the Output Compliance for your circuit. For example, for a 5V compliance, a maximum of 5 mA current will be allowed for a 1 kohm load.


  • I'm really sorry for late reply. I'm very busy and under pressured for my job.

    the output impedance should be greater than 100Kohm for 10KHz to 5MHz  and the current should set to 100uA or 1mA.

    I think to acheive a high impedance beyond 1MHz is not easy.


  • Hi Ali,

    The best example or reference for your application is the CN0151. See figure 3.


    The first AD8510 is a TIA to convert the current out of AD5443. So if you are in voltage mode, disregard the first OA. The computations are shown and explained the circuit note.

    Should you have additional concerns, don't hesitate to let me know.



  • Thank's for your attention and recommendation

    I've studied many papers about VCCS. (almost all papers )

    it is one of the first application notes that I had studied before  

    one of the worst drawback of such circuit is the mismatch between resistors.

    if you want a good accuracy it's better to use very accurate resistors (0.005% or better)  

    it's a good paper to talk about it: 

    I think AMP03 or INA105 are good choices for such application.

    so you need a unity gain feedback (I'd like to call it a moderate version)

    if you want very accurate VCCS it's better to use a DC stabilized feedback. (I'd like to call it a  advanced version)

    and ...


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