Post Go back to editing

constant current source application with ad8655

Category: Hardware

Hello, I am now ready to use ad8655 in a low noise constant current source application, in the debugging, at the beginning there is no C1, R2, ad8655 IN+ and IN- can not be equal, and then read some articles, indicating that C1, R2 should be added here, I add the resistance and capacitance value as shown in the figure, the voltage of IN+ and IN- is still not consistent, the voltage of IN- is greater than IN+, how to choose the peripheral resistance capacitance of the ad8655, I hope to solve the doubt, thank you!

Top Replies

    •  Analog Employees 
    Nov 13, 2023 in reply to licunxu +1 verified

    Hello ,

    Sorry for the delayed response. It appears that your circuit is not stable. My guess is that the output impedance of the operational amplifier interacts with the transistor's input capacitance…

  • Hi,

     will share his inputs shortly. 

    Thanks,

    Mae

  • Hello ,

    Just to make sure that I got that right, you want to know how to choose/calculate the values of C1 and R2 so that the voltage on both IN+ and IN- inputs will be equal or almost equal right? Though may I ask, by how much do the voltages on IN+ and IN- differ when there is no C1 and R2, and, when there is C1 and R2?

    Best regards,
    Paul

  • Yes,I want to know how to choose/calculate the values of C1 and R2 so that the voltage on both IN+ and IN- inputs will be equal or almost equal right. 

    when there is C1 and R2, C1=50pf, R2=500Ω, it is about 0.1V on IN+ and IN- differ. when there is no C1 and R2, I do not measure the voltage.

  • Hello ,

    On your schematic, R2 = 5 ohms. Are you originally referring to R4?

    Can you provide pictures of the signal on both the IN+ and IN- inputs when there is no C1 and R4, and, when there is C1 and R4? Can you also send your schematic file?

    Best regards,
    Paul

  • Hello Paul,

    yes, it is R4.

    I only use multimeter to measure the voltage on both the IN+/- inputs. There is no picture, and my schematics is show in figure that i post 

  • Hello ,

    Sorry for the delayed response. It appears that your circuit is not stable. My guess is that the output impedance of the operational amplifier interacts with the transistor's input capacitance which in turn creates a pole on the op amp's Aol curve and causes a rate-of-closure (ROC) of greater the 20dB. In general, to ensure stability, a minimum phase margin of 45° (I do recommend a higher phase margin than 45° but less than 90°) or a rate-of-closure (ROC) of 20 dB at fcl where the Aol plot intersects the 1/β plot is required. Note: You will have to do some simulations using a software like LTSpice in order to get these plots.

    In order to achieve stability, compensation techniques just like the addition of an isolation resistor (Riso), feedback capacitor (CF), and feedback resistor (RF) can be done.  

    Riso (R1 in your case) will cancel the additional pole formed by compensating the circuit and adding a zero on it. To calculate its value, the formula below can be used: 

    where:

    fzero = the zero frequency where the loaded Aol plot is at 20 dB

    Cload = input capacitance of the MOSFET (found on the transistor's data sheet)


    After the addition of Riso, if the circuit remains marginally stable due to a zero in the 1/β plot, an additional feedback capacitor (C1 in your case) can be added to bypass the transistor at higher frequencies and create a high frequency pole. In order to calculate CF or C1, the formula below can be used:

    where:

    fzero(1/β) = the frequency where the zero on the 1/β plot is located

    Riso = value of computed isolation resistor


    In case peaking occurs on the 1/β plot after the addition of both Riso and CF, adding a feedback resistor (R4 in your case) can help fix the problem. The peaking which causes instability will be more likely caused by the interaction of CF with the transconductance of the MOSFET. Adding RF or R4 will help flatten out the 1/β plot and isolate the feedback capacitor from the MOSFET. For the value of RF or R4, typically, it is usually in the range of 1k to 10k. You can simulate different values of RF or R4 from the range that I've given you and see which value will give you the desired response.

    I hope this helps.

    Best regards,
    Paul