Can anyone give me detail and schematics on reference votage generation scheme using OPAMP 741 for ADC0808?

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Answer 1

An op amp can only buffer a reference voltage. You still need a reliable source of reference voltage. I could recomend a device such as a LM336-2.5 . This will provide a 2.5V reference which you can then use the op amp to buffer or adjust using precision resistors as needed.