AD8251, AD8253 Model Convergence


I am attempting to use the AD825x devices as current sense and voltage sense instrumentation amplifiers in an analog control loop. I was unable to get the SPICE models for the AD825x to work with PSpice and LTSpice. While the circuit simulation was stable with an approximation of the AD825x (gain followed by a low pass filter), the actual circuit when built was not stable.

I understand that my approximation of the gain/phase response of the instrumentation amplifier was not detailed enough, the frequency response of the devices is critical to the stability of the circuit.

I need some help getting a basic circuit to converge with the actual devices. I have built a simple model below; however this model does not converge.

I get the following error (The full SPICE output is attached):

  These supply currents failed to converge:

    I(X_U1.EREF1)             =   -10.00GA  \   -10.00GA

    I(X_U1.EPSB1)             =   -10.00GA  \   -10.00GA

The basic idea of the application is shown below:

Any help would be appreciated. Thanks!

  • Hi Igor,

    Good day!

    AD825x SPICE models were tested in different simulation engine and one of them is PSPICE. I also ran a quick simulation base on your attached schematic and mine works fine. I ran a 1000ms time domain (transient) response for this circuit (all sims options are defaults). If you're still unable to converge your circuit, I suggest you try GMIN Stepping in the options, try raising ITL1 to 500, RELTOL to 0.01. Relaxing the simulation tolerances/limits can ease up the machine to run your simulation.



  • Igor,

      Usually when you get this kind of error message, it's because you have the symbol pin mapping wrong.

    Read the section in your simulator manual on how to connect a symbol to a subcircuit file.


  • Hi Guys,

    Thanks for your help. I re-made the parts and the simulation works (although it still does not converge every time.)

    My simulation is currently is shown below. For some reason I am seeing input bias currents of about 4uA (top plot) into U1. This seems a little high to me. It is producing an offset of a few mA due to the high input resistances (2nd plot from the top).

    Is this a simulation artifact? How can I improve the input currents? I tired running the AD8251 from +/- 15V with similar results. The datasheet quotes input currents in the order of 50nA.


  • Hello Igor,

    I'm glad you already ran your circuit. On your concern on the bias current, I'm going to run and check it and let you know if the model needs some tweaking.



  • Any model update on AD8251 ? I failed to run the spice simulation with AD8251 using ltspice.