I tried out op-amp level shifters on web. The ones recomended here as well but there is a problem when no input applied. The output voltage is no seperable from the case voltage applied. As i understand it is called op-amp biasing. I am trying to apply reference voltage with shift voltage but calculation isnt simple. So i am guessing there is a commoon way to do it. How can i sense opamp level shifter has no inputs or how to bias that shifter?

i am using that one (the popular one i guess)

And my biasing idea is like that:

enter image description here

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    \$\begingroup\$ post an example circuit of what level shifter you are using. \$\endgroup\$ – helloworld922 Jun 1 '14 at 15:48

If the input is disconnected, if that's what you mean by "no input", the output will not be midscale on the ADC. It will be indistinguishable from a 1.6V input.

You can deal with that by buffering the input (voltage follower) and adding a high-value resistor to ground on the input of the buffer, but you'd need power supplies higher than +/-5V for the buffer amplifier (how much higher depends on the op-amp used).

You can also apply a reference voltage other than ground to the left end of R1 in your top diagram, but it will have to be a negative reference voltage to shift the output above ground. That would allow a single op-amp with a single supply, but you need a negative reference voltage.

You could also add a resistor from the ADC input (right hand side of R? on your top diagram) to a positive reference, which would allow a single op-amp to be used again, but that op-amp would need a negative supply to allow the output to go to zero.

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  • \$\begingroup\$ Thank you. yes that is what i mean. all the answers require negative supply: That will change my design critically and cost will increase dramatically (because of isolated dc-dc converter). i wonder if is there a way without negative supply \$\endgroup\$ – erkan Jun 1 '14 at 17:32
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    \$\begingroup\$ Create a negative supply with a charge pump, eg. 7660 or LTC has some good parts. \$\endgroup\$ – Spehro Pefhany Jun 1 '14 at 18:22
  • \$\begingroup\$ i will check if i can find cheap voltage inverter in my country. I may try No:3 of Dave's solutions: youtu.be/g7b5YZENvjY?t=28m16s \$\endgroup\$ – erkan Jun 1 '14 at 18:32

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