AD829 stability

Hello,

Is the AD829 stable in the following conditions ? What is the Phase Margin ?

- CL=150pF capacitor load

- RL=~1kohms resistor load

- Vs = +/- 5V

- Ccomp = 15pF to inverted input

I attached the schematic (VEE = -5V and VCC = +5V).

Can I simulate the phase margin with PSPICE and how ? Is the model realistic in theses conditions ?

Thanks for your answer,

Mathieu

  • 0
    •  Analog Employees 
    on Oct 22, 2012 5:10 PM over 8 years ago

    Hi mbart,

    I apologize for the delay in response. I've forwarded your query to the engineer who worked on the SPICE model for AD829 regarding simulations to get the phase margin for your circuit. However, I had a spare board here and I tried out your circuit using just one of your branches as input (tried the one with the lowest gain). With a 15pF compensation cap, there is oscillation at the output (see below).

    I switched the compensation cap to 68pF (which is recommended in the datasheet for a gain of 1), and I lost the oscillation. On bench, even with the resistors set to have a gain of 0.5, the output is still stable (see below).

    If you can still make amends to your circuit, I would recommend that you go instead with a compensation cap of 68pF. The only other option would be to increase your gain to get rid of the oscillation.

    I hope this helps!

    All the best,

    Kris

  • Thanks a lot for your answer.

    I will do the same test to try to reproduce this oscillation.

    Could you tell me how did you choose 68pF Ccomp capacitor ? How could I choose the "good" capacitor value ?

    When connecting the Ccomp to the inverting input, the datasheet doesn't mention the capacitor to choose. There is some figures where Ccomp = 3pF/ 4pF or 3pF with inverting gains of -1V/V to -4V/V (pages 14 and 15).

    Ccomp = 68pF at the gain of +1V/V is mentionned when Ccomp is connecting to ground (Table 4 page 13).

    Do you have some news about stability simulation ? I hope I can use the model to choose the "good" capacitor value for stability, or that you will help me. Because when connecting Ccomp to inverting input, the capacitor value vs gain is not mentioned in the datasheet, contrary to the connection of Ccomp to ground (Table 4 page 13).

  • 0
    •  Analog Employees 
    on Nov 8, 2012 11:30 PM over 8 years ago

    Hi Mathieu,

    I apologize, in my experiments, I was connecting the compensation capacitor to ground instead of at the inverting input as in your circuit. I just tried out your circuit connections on bench, and I am seeing oscillation at the output. I just heard back from the engineer, and unfortunately, the AD829 is a very old part and its SPICE model cannot be relied on to model stability properly. To be more flexible with your compensation options, would you consider using the shunt configuration? Increasing your gain would also work.

    Could you tell me more about your application? I assume that you are trying to get more slew rate out of your circuit since you are using the current feedback configuration for compensation. If it's not in your list of options to use shunt compensation or to increase your gain, maybe you could provide the requirements for your amplifier so that I could try to find an amplifier that will work for your application.

    Regards,

    Kris

  • Hi Kris,

    Thanks for you answer and I apologize for my approximative english.

    I can't change :

    - the gain of amplifier

    - the amplifier (only the AD829)

    - the type of compensation, because i need slew rate (above 60V/µs as a minimum)

    The only thing I can change is :

    - first of all, the value of Ccomp capacitor,

    - and if it is still not stable :

         - the value of capacitor load C61

         - or the value of resistors but without changing the gain

    Are you sure you tested the circuit with a capacitor load C61 = 150pF ?

    Could you please provide a screenshot of the oscillation ?

    Regards,

    Mathieu

  • 0
    •  Analog Employees 
    on Nov 9, 2012 9:02 PM over 8 years ago

    Hi Mathieu,

    maybe you can try adding a small resistor at the output to snub the capacitive load.

    This should help stabilizing the part.

    Charly