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My hall effect sensor (WCS1700) outputs 2.5v at zero current, I need to remove this offset voltage, and also amplify output.

I utilized both the opamps of LM358, first stage (A) to amplify signal and second(B) stage opamp to remove offset voltage. Under simulation it works as expected but in real world etched pcb not working.

Side A opamp can amplify the signal successfully but B side isnt able to remove offset, basically I get same output as non-inverting(B side) voltage.

Please look my layouts and simulation result, kindly someone help I have very limited electronics knowhow.

https://ibb.co/dJ3VNg2

https://ibb.co/5v6VmPY

https://ibb.co/GPYMDS3

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    \$\begingroup\$ What are the power supplies of your op amp? If you're trying to go to 0V with a single side power supply, that's often not a good idea for many op amps. Also, can you include all the images in your question instead of linking out to them? \$\endgroup\$ Aug 30, 2023 at 14:53
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    \$\begingroup\$ LM358 isn’t rail-to-rail. \$\endgroup\$
    – winny
    Aug 30, 2023 at 14:55
  • \$\begingroup\$ @ScottSeidman the other images have 5v input to LM358, I am using mobile browser to post, and image url paste somehow didn't work. \$\endgroup\$
    – duckduckgo
    Aug 30, 2023 at 15:33
  • \$\begingroup\$ @winny my input voltage is hovering around 2.5 +/-0.1 v for either side of opamp, do i really require rail to rail \$\endgroup\$
    – duckduckgo
    Aug 30, 2023 at 15:41
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    \$\begingroup\$ If you want 2.5 V input = 0 V input, you need an opamp which is rail-to-rail on the output, or +- supply. \$\endgroup\$
    – winny
    Aug 30, 2023 at 16:31

1 Answer 1

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You're providing a single-sided supply to an LM358.

The data sheet states

9.2.1 Design Requirements The supply voltage must be chosen such that it is \$>\$ application scales a signal of ±0.5 V to ±1.8 V. Setting the supply at ±12 V is sufficient to accommodate this application.

However, the Characterics on page 1 say

Common-mode input voltage range includes ground, enabling direct sensing near ground

So, I think on the input range, we're OK (but close)

However, the output is a different story. The tables (Table 6.5) show that for a 1 mA output, you can't get closer to the negative rail than 0.75 V nominal.

An LM358 can do what you want it to do if you provide a dual rail power scheme. With your power scheme, you need to use an op amp that is rail-to-rail on both the input and output (you need to read the data sheet to make sure) -- but even then you may not get all the way to zero on the output, just within some tens of millivolts.

On every data sheet for every op amp, there will be information on what allowable inputs and outputs are for given power schemes. FWIW, I think mistakes along these lines are one of the most frequent posts for op amp design issues here. There are some hoops you need to jump through when going from ideal op amps to things you can actually buy.

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  • \$\begingroup\$ thank you for detailed answer, Now i understand root of the problem, but as my signal even post amplification stage remain approx 10mV, I am doubtful using an opamp can work \$\endgroup\$
    – duckduckgo
    Aug 31, 2023 at 0:31

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