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I have an op-amp design same with the picture seen below with TL072.I have an analog sensor that goes as Vin. I am giving 3.3V from an arduino to power it. And reading values from an analog pin.

The problem is that the output gives constant 1.2 V while sensor and resistors are disconnected and 2.3V while they are connected.

r2=220 ohm| rf=2.2k ohm| sensor gives output between 0-200 mV

 Image taken from http://www.electronics-tutorials.ws/opamp/opamp_3.html

*Image taken from http://www.electronics-tutorials.ws/opamp/opamp_3.html

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    \$\begingroup\$ You have chosen the wrong op-amp. If you read the datasheet you will see that the TL071 can only get within a volt or so of the supply rails. Yours can't move either way. You are looking for low-voltage op-amps with rail-rail output capability. \$\endgroup\$
    – Transistor
    May 15 '16 at 8:50
  • \$\begingroup\$ Indeed the TL071 is not suited for such a low supply voltage, get a TLC271 or an MCP601 or MCP6001 instead. These are designed for supply voltages of 5 V and lower. \$\endgroup\$ May 15 '16 at 10:11
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You are trying to measure a voltage that is close to the negative supply rail (so you need a single-supply op-amp), you have a low supply voltage (so only some low voltage op-amps are suitable), your voltage is rather low (so offset voltage matters). The output needs to swing near the negative rail (so RRO or single supply). If you increased the supply voltages (say +10/-5) to get the TL072 to work you'd run into the requirement to keep the input voltage to the ADC within approximately the range of 0-3.3V.

Your TL072 is thus unsuitable for a myriad (or maybe even a plethora) of reasons.

Follow @Fakemoustache's suggestion- and used something like a MCP606. It's also compliant with any desire you may have to avoid very tiny packages- it's available in 8-pin DIP. Offset voltage is max +/-250uV so offset error in the output will be less than 3mV.

I also suggest you increase the values of your feedback resistors by perhaps 10:1 (22K/2.2K) or maybe 49.9K/4.99K 1% because your values will load the op-amp output a lot and cause more gain error than necessary. Your gain in any case will be ~11 so you will get 0-2.2V out for 0-200mV in.

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  • \$\begingroup\$ Will TLC271 work instead of MCP606 because MCP opamp series isn't avaliable in where i live? \$\endgroup\$
    – bcan
    May 15 '16 at 20:15
  • \$\begingroup\$ It's worse in about every way, but it should function, with the exception that it might have trouble making it all the way up to 2.2V especially at high temperature. Try 100K/10K feedback resistors to give it a fighting chance. Offset can be as bad as +/-10mV depending on type, so it could have 220mV offset at the output (40x worse). \$\endgroup\$ May 15 '16 at 21:30
  • \$\begingroup\$ Lastly, how much ampere would it drain with this setup? I have another component connected to arduino ( a zigbee module) and a powerbank that powers arduino with 5V and 2A. \$\endgroup\$
    – bcan
    May 16 '16 at 7:04
  • \$\begingroup\$ Not much. Look up the supply current for the op-amp of your choice from the datasheet, and add the maximum output voltage divided by (R2+ Rf). \$\endgroup\$ May 16 '16 at 7:19
  • \$\begingroup\$ I changed op-amp to TLC271, tried with 220 ohm/2.2k ohm and 220 ohm / 22k ohm resistors and they both gave result near 0. Should i take higher resistors to make it work? \$\endgroup\$
    – bcan
    May 20 '16 at 17:11
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kindly increase the supply voltage as suggested in the datasheet

http://www.ti.com/product/TL072

(Note : it requires two supplies one +ve and another -ve)

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  • \$\begingroup\$ I will try add a battery to opamp input and output but should battery's ground connected to arduino's ground? \$\endgroup\$
    – bcan
    May 15 '16 at 7:50
  • \$\begingroup\$ yes connect the battery to arduino ground and op-amp +ve,-ve supply (not input) terminals. \$\endgroup\$
    – Jasen
    May 15 '16 at 7:56
  • \$\begingroup\$ Sorry, I tried to mean +ve, -ve. \$\endgroup\$
    – bcan
    May 15 '16 at 7:57
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Depending on the accuracy you need, it might be simpler to use a transistor amplifier here. Maybe you can calibrate the result in software and avoid using a more accurate and more expensive op-amp solution.

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