Biased single-source relaxation oscillator not starting

Question

Vcc is 5 V , first op-amp LM358 provides 2.5 V ground for easiest possible relaxation oscillator build with MCP661 (2.5/5.5 V supply, 60 MHz operational range). I wanted to use it to measure values of capacitors. Yet for some reason it doesn’t start but hangs with the output at 2.5 V when connecting any capacitance.

Please point me towards what I might have done wrong.

analog low voltage measurement operational amplifier oscillator

Please point me towards what I might have done wrong.

Andy aka · Answer

You are over-complicating things and have too much hysteresis. A single rail (and single op-amp) relaxation oscillator like this will do the job: -

Notice that R3 and R4 provide the "loose" mid-rail that can be superimposed with the positive feedback from the op-amp output. This circuit has significantly lower hysteresis too because ofthe effects of both R3 and R4.

user287001 · Answer

You have too large hysteresis. Make R3 50% smaller OR double R5 and compensate by doubling R4 (only approximate compensation)

EDIT: The constant 2,5V output fits to the case that you have accidentally exchanged the inverting and non inverting inputs of U1. Quadruple check this from alternative data sources.

Biased single-source relaxation oscillator not starting

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