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INA333: SPICE noise simulation with different results

Víctor Jimenez
Prodigy 10 points
Part Number: INA333
Other Parts Discussed in Thread: TINA-TI, , OPA333

Hello.

I'm doing some simple simulations in Pspice and TINA-TI (using the Wheatstone bridge example) and have found the following discrepancy:


1.- Simulating with TINA-TI and using the default model of this library (in Spice Macros -> Instrumentation Amplifiers ->INA333) the result is the following:

2.- If I use the simulation model INA333 TINA-TI Reference Design (Rev. F) SBOC264F.TSC (1395 KB) - TINA-TI Reference Design (https://www.ti.com/lit/tsc/ sboc264) and I modify it to make it the same as the previous simulation then I get these results:

Conclusions:

The results of the first simulation are more similar to the hand-made calculations.

In the second simulation the results are higher than in the first.

Using PSPICE with the INA333 simulation model PSpice Model (Rev. G) SBOM382G.ZIP (26 KB) - (www.ti.com/.../sbom382) the result is identical to the second simulation.

Changing the gain to G=100 Rg=1k in both situations, the results are also different.

Ask:

Which simulation is closest to reality? Which simulation model is more reliable?

Since the results are so different and considering that the simulation model provided by TI is more up to date, it should be the most appropriate but it is different from the usual hand-made calculations.

I attach the simulation files.

ina333 tina ruido.TSCina333 ruido.TSC

  • 0
    TI__Guru 64290 points

    Victor,

    Most of the simulation models under Tina's Spice Macro tab are outdated and should not be used - you should always download up-to-date latest version of the macro-models (in case of INA333 Rev G) from the product folder.

    If you you look at the netlist of the first macro you simulated, it shows version 1.0 creation in May 2011. Actually, it's a preliminary simplified model which later has been replaced by our brand new macro-model architecture called: Green-Williams-Lis model, which we have been using for the past decade for all new op amps and INA's models.

    All of the versions (E, F, G) under product folder use our modern macro-model and show the input voltage noise spectral density of 79uV/rt-Hz.

    One last thing, the INA333 maximum total supply voltage is 5.5V (or +/-2.75V), thus you cannot use +/-5V supplies.  Also, Wheatstone bridge resistors generate thermal noise and are additional noise source not included in the INA333 datasheet.  

    ina333 ruido edited by Marek.TSC

  • 0 in reply to Marek Lis
    Prodigy 10 points

    Hi Marek,

    You're right, I haven't realized the power input range. Although for this simulation it has no effect.

    I had also checked the versions of the models and seen that the most current one is the one from the second simulation (even the PSpice version is 1.3).

    However, using your review of the circuit, the levels shown in the simulation are significantly higher than what I do in the calculations (file attached). For:

    • Total Output Noise Voltage Spectral Density
      Simulated= 79uV/√Hz
      Calculated=50uV/√Hz

    • Total RMS Output Noise Voltage
      Simulated= 2.13mVrms @3.5KHz (rule of thumb: 10 times signal bandwidth)
      Calculated=1.17mVrms

    I've been trying to find out why this happens but I can't find where the error is. I don't know if something is missing in those calculations, or if the simulation model for the noise is not realistic (I don't have enough knowledge to determine this).

    Any help is welcome.

    Thank you.

    3513.INA333 ruido v1.xlsx

  • 0 in reply to Víctor Jimenez
    TI__Guru 64290 points

    Victor,

    Please see my comments below:

    • Total Output Noise Voltage Spectral Density
      Simulated= 79uV/√Hz
      Calculated=50uV/√Hz

    INA333 uses in first stage two OPA333 with Vin_noise of 55nV/rt-Hz, thus the INA333 input noise should be: Vin_noise=~55nV/√Hz*√2=~78nV/√Hz.

    Thus, the calculated output noise =1000*78nV/√Hz = 78uV/√Hz but because of the additional resistor thermal noise the output noise is 79uV/√Hz.

    • Total Output RMS Noise Voltage
      Simulated= 2.13mVrms @3.5KHz (rule of thumb: 10 times signal bandwidth)
      Calculated=1.17mVrms

    The total Vout_noise = Gain*Vin_noise*√BWn = 1,000*79nV/√Hz*√(350Hz*1.57) =~1.83 mVrms which I believe is an approximately correct value.

    For the G=1,000 the simulation shows -3dB bandwidth of 506kHz (see AC gain sim below) instead of 350kHz (I believe that's a modeling error)  

    Simulated Vout_noise = Gain*Vin_noise*√BWn = 1,000*79nV/√Hz*√(506Hz*1.57) = 2.23mVrms matches simulation but I do not believe is correct.

    4604.ina333 ruido edited by Marek.TSC