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LMV358A: Astable Multivibrator - Odd Behavior

Matthew Anauo
Prodigy 140 points
Part Number: LMV358A

Tool/software:

Hi,

I am using the following circuit as an astable multivibrator, with the output frequency being a function of the varying resistance of an RTD (P/N is Vishay NTCS0402E3103FLT, represented by {Rt} in the SPICE capture):



The simulated behavior matches test measurements for higher sensor resistance values, but diverges at lower resistances. When I replace the LMV358A model with a generic spice op-amp, the shape of the response aligns with lab measurements (although offset by a considerable amount):

I'm trying to understand the cause for this, is it a simulation artifact (seems unlikely - I simulated the circuit in TINA with the LMV358A model and got identical results to what I derived in LTSPICE), how to explain the discrepancy to the lab measurements.

  • 0
    TI__Guru*** 149226 points

    Hello Matthew,

    LMV358A is not especially fast and is not a comparator. Slew rate and over load recovery time (analogous to propagation delay) will play a role is output frequency. To get more accurate results, use faster comparator and make sure capacitor is accurate and linear; meaning low voltage and temperature modulation and low tolerance in baseline value.  Also your first image did not upload properly, I can't open it.