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OPA2626: TINA-TI Simulation Error

Genius 9880 points
Part Number: OPA2626
Other Parts Discussed in Thread: LM7705

Hi Team, 

Customer is encountering strange behavior during the OPA2626 simulation. Below is the details.

"
A simple voltage buffer transient simulation was configured with the 0 initial value option. I can find opa2626's strange behavior. Please refer the included picture.

I want to use this opa2626 for 1V reference voltage buffer. I think there shouldn't be any problems with my circuit. If I choose 1k ohm load, there is no problem. Is there anything wrong I missed?

"

Thank you in advance.

Regards,

Maynard

  • Hi Maynard,

    When the source signal is at zero volts with a single supply configuration, you are violating the output swing limitation of the amplifier.

    Best,
    Sam

  • Hi Maynard,

    strange Thinking

    When you intend to go all the way down to 0V with the input signal, you should think about using the LM7705:

    maynard_opa626.TSC

    Kai

  • Hi Sam,

    Customer have following questions regarding this. Below is the details.

    "When the source signal is at zero volts with a single supply configuration, you are violating the output swing limitation of the amplifier." 
    Would you please explain about "violating the output swing limitation" more kindly?
    OPA2626 has rail to rail output and input common mode including negative rail. I think there is no violation issue of input and output common mode range.
    Also, this issue occurs in dual supply configuration too.
    Please note that the input voltage is DC 1V. This circuit with OPA2626 is configured as the voltage reference buffer for fast sar ADC. 
    Actually, This is not a critical issue but I'm just curious about this behavior and I want to find my mistake.
    The only transient analysis option that shows no issue is "Calculate operating point"
    Thank you in advance.
    Regards,
    Maynard
  •  Hi Maynard,

    We generally don't recommend using the 'zero initial values' option for a circuit like this because it can cause erroneous startup conditions in the model behavior. That option is best reserved for oscillators or other circuits without a DC operating point.

    The issue I was referring to in my original answer is that when you force the excitation signal to start at zero volts, you are asking the amplifier to output zero volts. This is a violation of the output voltage swing spec, which may be related to the load dependent behavior you described in your first post.

    You can see if you start within the allowable range and drive the output to the negative rail, it will show the swing limitation.

    I have spent a fair bit of time looking at both the single and dual supply modes, and it appears that this circuit is not suited for the 'zero initial values' option. This option sets all voltages and currents to zero, including the op-amp supplies, so on startup you have a condition where the amplifier and the excitation are both trying to turn on. 

    It is best to allow the simulator to calculate a DC operating point for a DC unity gain buffer circuit such as this.

    I hope you find this answer helpful.

    Best regards,

    Sam

  • Hi Maynard,

    do not rely too much on all these simulations. It's only a starting point and a useful tool but not the holy grail. The most simulations are helpful but not all. Some of them are just erratic or even ridiculous Relaxed

    Kai

  • Hi Sam and Kai,

    Thank you for your support.

    This resolved the issue.

    Regards,
    Maynard