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enter image description hereenter image description hereI have a circuit using LM358N and LTC6240 op amps. The supply is 0-5V and I have used a virtual ground circuit, where 2.5V is my reference. I am happy with my filtered and amplified signal and now want to connect it to the external interrupt of a PIC18F4550. I therefore need to transpose my ac signal from -2.5 - +2.5V to 0-5V

I have tried summing 2.5V from a voltage divider with my signal but get a 0V DC output.

I'm quite perplexed as it seems a simple procedure.

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  • \$\begingroup\$ Schematic of your summing amp? \$\endgroup\$ – hwengmgr Mar 26 '15 at 22:17
  • \$\begingroup\$ Hi hwengngr,the schematic is up now. \$\endgroup\$ – Leon Ellis Mar 26 '15 at 22:34
  • \$\begingroup\$ looks like you are missing a feedback resistor around U6. with no feedback, the output of the opamp will slam to one of the rails. \$\endgroup\$ – hwengmgr Mar 26 '15 at 22:49
  • \$\begingroup\$ actually the feedback does not have to be a resistor. Just connect the output back the "-" input. remove the ground from the "-" input. then you have a non-inverting, gain of 1 amp. But if you are trying to feed this into the interrupt of the processor, then you want a digital signal, don't you? then you want to have a comparator (like an LM339) with a threshold voltage to trip at. not sure what good feeding an analog waveform into the interrupt pin would be. \$\endgroup\$ – hwengmgr Mar 26 '15 at 23:04
  • \$\begingroup\$ My idea is to catch the rising edge of a 5V square wave (achieved by saturating my sinusoidal input). Any voltage above 4V will trigger the interrupt. \$\endgroup\$ – Leon Ellis Mar 26 '15 at 23:29
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First: your schematic has an error. The ground symbol under U1a probably shouldn't be there - the bus that it is tied to comes from your virtual earth supply.

I assume that U6 is your attempt to fix your problem. That isn't the correct solution.

Look up the phrase "differential amplifier" on Google. You can configure one op-amp section as a simple diff-amp using only the op-amp and 4 resistors. You simply subtract the virtual earth rail from your output and you have the result you want.

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