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Inverting Amplifier problem

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engineer123

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Hey Everybody,

I had PIC works as ADC and it works very well with positive voltages the problem is that I have to convert from A2D through the PIC negative voltages so what I did simple design of inverting amplifier which converts the negative voltage to positive voltage and the output of the amplifier connected to the ADC of the PIC the amplifier works fine as expected , but the ADC result is not stable it keep changing.
 

The problem is probably related to how you designed the amplifier.
 

here the design, attached , but i used 10k not 1k its matter of ratio , please have alook at the attached file and let me know if there is problem with it , and the amplifier that i used , OPA134/BB

thanks
 

OPA134 isn't capable of rail-to-rail input operation, particularly common mode range down to negative supply as required by the design.
 

About half way down page 2 of the data sheet is "Input Voltage Range". This is a traditional op amp, not one of the new "rail-to-rail" parts. The input voltage range can be as small as the total power supply voltage minus 5V. Since your total power is 5V, that's a problem. All major op amp vendors have parts better suited to what you are doing, or you can use this amp but run it from +5 and -5 supplies. Also, Microchip makes op amps for these kinds of applications.

ak
 

Hi;

analogkid maybe you right , but i just tried another thing connected the output of the opamp capacitor to the ground sort of filtering , for about 100uf , and i think it works now, i dont know how i missed this thing before
 

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