Voltage divider with differential amplifier.

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Cloudy221

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Hello everyone.
I have to create an offset for an AC range of 90 - 250 V RMS. to fed into an A/D. I have got the voltage divider in place it is just the dif amp that I cant get right. Once I connect the amp, the voltage divider changes its output due to the amp acting as a load. For the life of me I cannot get the diff amp to offset my signal by 2.5V. Can anyone help me? What resistor values do I use for the diff amp?
Any help at all would be awesome.
 

If you lean back and review your post, you'll notice that it misses most necessary information for a reasonable answer.

- voltage divider resistance values
- type/circuit of differential amplifier
- available supply voltage

As a small hint, connecting the voltage divider's foot point to 2.5V can be sufficient to achieve what you want, without any amplifier.
 

Sometimes in practice it’s more difficult to implement such a summing circuit , you may use additional OA, see the diagram attached.
There are the two situations, first possibility implementing using a 2.5V voltage reference and second using any voltage reference we are able to provide (U ref> 2.5V) and adjusting for the necessary value.
 

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