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THS4511: Influence of supply voltage to the simulation

Part Number: THS4511
Other Parts Discussed in Thread: THS4513

Hi,

I want to simulate a differential second order high pass filter like in the example attached. The circuit is designed to reach the -3dB point at 10 MHz and gain should be zero. Everything looks like expected when i choose VCC = 5.5V. However lowering the supply voltage to 5V has a high impact on the simulation. The output level of 0dB is never reached. Is this behaviour to be expected or is it a model error?

Thanks in advance

mfp_second_order_highpass.tsc

  • Hi Olaf,

    L1 and L2, what role do they play?

    Kai

  • Hello Olaf and Kai,

       I believe the inductors are there to set a DC operating point for the simulation to work. You could also do this by changing the capacitor's Rpar from infinite to 1G (double clicking the capacitors to bring up the option). I played around with the circuit a bit, and it looks like also lowering the Vcm from 2.5 to 2 gives the expected output. Replacing the THS4511 with a similar part THS4513 also gives the correct output. I need to look into this more and get back to you by tomorrow

    Thank you,

    Sima

  • Hi,

    Yes, the inductors are used to set the DC operating point. Thanks for the hint to change the capacitors parameters directly. I can confirm, that the combination of 5V VCC and 2V Vocm does seem to work properly or at least the AC simulation looks reasonable. A closer look to the datasheet states that for the THS4511 the common input has to be in the range of -0.3V to 2.3V so is this my mystake (sorry if so)? The THS4513 on the other hand supports a range of 0.75 to 4.25V. The simulation with Vocm set to 2,3V doesn't look well though.

    The reason why i chose the THS4511 is that its unity gain stable. Maybe i could ask another question. How would one proof by simulation that this circuit is stable?

    Regards

  • Hello Olaf,

    Talked to my team, and it looks like THS4511 spice model is in a middle of a rework; since, there are some known model issues for that part. THS4513 is also unity stable with as you said a higher common-mode input range. Would you be able to switch to this part? The mistake is not on you, the common-mode input range should not be causing a problem here. There is a different spec for Vcm (or Vocm) which your circuit is within the acceptable range. I also do not think it is a stability problem since it starts working at different supply ranges. I will make a stability circuit though as a reference, and will get that to you by tomorrow. 

    Thank you,

    Sima 

  • Hello Olaf,

       Here is a stability simulation of the THS4511 and THS4513 with your application and component values. Let me know if you have any questions on the simulation. I will also attach some amplifier stability references: FDA Stability Video and MFB Stability Article (PDF download). 

    Thank you,

    Sima 

    MFP_HighPass_StabilityUpdated.TSC