extract positive voltage to read adc

[electros6]

HI guys,
           I have a circuit that using opamp using dual supply +- 5v. I want to read the output of the opamp using adc . since the opamp swings between the positive and negative rail ,please give any idea to built a circuit that allow positive voltage and block negative voltage.

[fcb]

Diode....

Or perhaps you'd get a better response if you describe your circuit in more detail, what the opamp is doing, what signal you are processing, etc...  maybe a sketch as well.

[electros6]

I just try to measure the intensity of the light, so I used the photodiode and current to voltage converter and an inverting amplifier and an offset adjusting stage. schematic attached

[Jeroen3]

"Convert" the +-15 Volts to 0-30Volts by putting your adc measurement circuit ground at -15 instead of 0v. Simple right?

Power the ADC from +15 and -15, this indeed requires some ldo or smps depending on the load. Add an amplifier with gain of 1/5, assuming the ADC is 5V. And you're almost done.
You'll still need anti-aliasing filtering and such necessities with adc's.

Expensive solution would be to get an bipolar ADC with such high voltage inputs. They exist.

Also, nowadays you can get pretty good adc's for a single power rail (eg. mcp600x). I would avoid dual rail unless it is absolutely necessary, and even then I'd create a virtual ground at Vcc/2 using an opamp.

[fcb]

Rework the resistors around your opamp to lower the maximum gain and re-centre your output to 0 to 3v3 or 0 to 5v.

 

[kizzap]

alternately you could use a differential amplifier and bias the output voltage to half full-scale.

and another thing: if you are expecting an OP-AMP to go rail-to-rail, you /will/ have a bad day. AFAIK there are /no/ op-amps which will go to the rail voltage. they will get close, but not exactly there. Put some overhead in your system if you can.

[Jeroen3]

Yes, you'll have severe non-linearity in the lower and upper 50mV. Same applies to your ADC, so there is no big deal.