I want to convert the output voltage of an ECG (-5 to +5) to 0 - +5 volt for ADC. Should i use Op amp ? What is my problem in this picture? I simulated it in Proteus ... but when input voltage is higher than 4 V or lower than -4 V Vout gives an unexpected value. enter image description here

Below is another simulink that have same problem ... enter image description here

My idea to make this level of voltage is (Vin +10)/2 that give the range 0-5 V.

  • \$\begingroup\$ Please use proper capitalisation and punctuation as required in standard English for this site. Question edited. \$\endgroup\$ – Transistor Jan 23 '16 at 11:53
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    \$\begingroup\$ The 741 opamp can not swing to negative rail. Find a better opamp. \$\endgroup\$ – Transistor Jan 23 '16 at 11:54
  • \$\begingroup\$ look here olimex.com/Products/Duino/Shields/SHIELD-EKG-EMG \$\endgroup\$ – Marko Buršič Jan 23 '16 at 12:12

The 741 can't accept an input that's too close to the negative rail. The data sheet says the input range is +/- 13V when the power supply is +/- 15V. Not sure about when the negative power supply is 0V but I doubt it will work the way you expect with a 0V input. Similar story on the output side. That would explain the strange numbers you're getting below -4V input. Don't know what's going on with the +4V input since there would seem to be plenty of headroom with your +12V supply. Does the simulation work if you power the 741 with +/- 15V? It looks basically correct if you assume that the op amp is ideal.


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