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I'm building a device to acquire current data through an appliance. The schematic can be seen below:

enter image description here

The problem is that I need this measurement to be step up tenfold, but I added a 2.5V reference just to make sure that the measurement stays within the 0V-5V range.

If I just use a gain op-amp, then the offset is also going to be step up tenfold, and the new offset would be 25V right?

How can I fix this?

in response to comment by @Neil_UK
Do you mean something like this?

enter image description here

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  • \$\begingroup\$ Why are you not using an in amp with a reference input? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Jul 26, 2016 at 18:18
  • \$\begingroup\$ When you say "outlet" do you mean a mains power socket? When you say "isolation" do you mean this device should prevent electrocution of users handling the Arduino circuit? \$\endgroup\$
    – The Photon
    Commented Jul 26, 2016 at 18:24
  • \$\begingroup\$ Because I need an isolated in-amp for safety \$\endgroup\$
    – William
    Commented Jul 26, 2016 at 18:25
  • \$\begingroup\$ Precisely, The Photon \$\endgroup\$
    – William
    Commented Jul 26, 2016 at 18:26
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    \$\begingroup\$ You might want to draw supply pins 1 and 4 on the left hand side of the package, to make it clear what the isolation function is! \$\endgroup\$
    – Neil_UK
    Commented Jul 26, 2016 at 20:47

1 Answer 1

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As the ISO130 has a common mode output of about 2.4v, and low impedance outputs that can supply 1mA, the following diff-amp will work

schematic

simulate this circuit – Schematic created using CircuitLab

This will give you 10:1 differential gain following the isolation amplifier. The voltage between ADC input and ADC ref will be 10x that between the OUT +/- pins. The ADC ref terminal can be taken to any convenient voltage that's good for the ADC.

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  • \$\begingroup\$ thanks, Neil_UK! In reality, I'm using an AMC1200/B Fully-Differential Isolation Amplifier. But I believe your idea should work for it as well \$\endgroup\$
    – William
    Commented Jul 26, 2016 at 21:32
  • \$\begingroup\$ Shouldn't the reference be at Out -? \$\endgroup\$
    – William
    Commented Jul 26, 2016 at 22:23
  • \$\begingroup\$ @William why would you want to do that? Do you know how a diff-amp like that works? Have you read the 130 data sheet (though I take no responsibility for being told by the OP after my reply that it's a 120, not a 130!)? I've explained in my answer that the 10x output occurs between the ADC input and ADC ref pins. \$\endgroup\$
    – Neil_UK
    Commented Jul 27, 2016 at 6:09

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