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I need to convert a 4v peak to peak signal (-2v to +2v) into a 0 to 3.3V signal to get it into my ADC on LPC 11u24. I have tried several designs but the problem is that all of the designs I found are based on single rail op amps. I only have dual supply op amps. Any suggestions?

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  • \$\begingroup\$ When you say 2v p-p do you mean bipolar like -1v to +1v (swinging above and below ground level) or just 0-2v ? \$\endgroup\$
    – alexan_e
    Jan 30 '14 at 17:00
  • \$\begingroup\$ You could try shifting the signal by adding DC using a summing amplifier with unity gain. \$\endgroup\$
    – Pyxzure
    Jan 30 '14 at 17:11
  • \$\begingroup\$ Oh Sorry I meant -2v to +2v \$\endgroup\$ Jan 30 '14 at 17:30
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    \$\begingroup\$ I only have dual supply op amps Why is that a problem preventing you from using the solutions you found for single rail opamps? \$\endgroup\$
    – alexan_e
    Jan 30 '14 at 18:26
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Based on this post, a summing amplifier could work. Adjusting the values, the schematic would be as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

The output is calculated just like a non-inverting amplifier. Thus, when the sine wave is +2V, the terminals of the op-amp are at 2V. Since

\$V_O = V_I(1+\frac{R_4}{R_3})\$

the output will be 3.3V. When the sine wave is -2V, the terminals will be 0V, since the two voltage sources, \$R_1\$, and \$R_2\$ act as a voltage divider. Then the output will also be 0V.

The caveat here is that you need a rail to rail op-amp, or supply voltages that are less than 0V and greater than 3.3V. Otherwise the op-amp will clip the output.

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This will level shift your +/-1V signal, however it requires a low impedance source and you lose a couple (1.7) bits of resolution.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Can you explain the resolution loss? \$\endgroup\$
    – dext0rb
    Jan 30 '14 at 17:26
  • \$\begingroup\$ The network above gives you a -3.3 to 3.3V range (6.6V p-p) as your signal is only 2V p-p, you lose \$log_2\$(3.3) bits. \$\endgroup\$ Jan 30 '14 at 18:03
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Use a non inverting amplifier with gain as 1.65.

This link could help you (http://www.instructables.com/id/Non-inverting-amplifier-with-uA741/?ALLSTEPS)

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