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I have a very small AC signal from a receiver coil of Metal Detector that I want to provide some DC offset so it can be handled by a microcontroller after it is amplified. I came up with this solution. Will it work?

Added detailed schematics

Schematics

Note that INA129 has on-board gain resistor INA129

Using the 5k POT, I can adjust offset and can also compensate for voltage drop of diode D3 on ADC2, ADC1 can be ADC or timer interrupt to analyze signal while ADC2 can detect changes in amplitude of receiving coil

INA129-HT Datasheet

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    \$\begingroup\$ If this is the receive coil amplifier of your metal detector, have you taken the opportunity to make the receive coil resonant with tuning capacitors? \$\endgroup\$ Commented Feb 25, 2022 at 12:02
  • \$\begingroup\$ Hi @Andyaka, yes I have, resonant frequency for receive coil is 6k and for tx coil, resonant frequency is set 6.1k because i heard it should have that much difference idk why, works tho \$\endgroup\$ Commented Feb 25, 2022 at 12:15
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    \$\begingroup\$ So, is the 6 kHz voltage in your picture connected directly to your coils? You need to show power rails to the op-amp and choose an op-amp to get a meaningful answer. You should also state what DC output level is needed by your microcontroller's ADC. You also need to recognize that the AC signal will only have a gain of 2 and that the circuit you propose doesn't easily allow the gain to be adjusted. \$\endgroup\$ Commented Feb 25, 2022 at 12:52
  • \$\begingroup\$ @Andyaka updated my schematics \$\endgroup\$ Commented Feb 25, 2022 at 13:37

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The proper way to do it is this: -

enter image description here

Problems with the original circuit: -

  • V-IN cannot be connected to ground if you are connecting V+IN to an offset
  • The offset at V+IN would be amplified by the InAmp gain
  • Noise from the 3.3 volt supply would get amplified by the InAmp gain.
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    \$\begingroup\$ There are 2 things a software dev like me motivated me to make a metal detector, 1. my detector actually working, 2. you being here for help, much appreciated @Andyaka \$\endgroup\$ Commented Feb 25, 2022 at 14:00
  • \$\begingroup\$ I still have doubts about the sanity of my design, being a hobby project its nothing too serious but do you think I am headed in the right direction? @Andy and how good is your method of using ferrite piece for fine tune balance of coils you mentioned in one answer? should I use it? \$\endgroup\$ Commented Feb 25, 2022 at 14:03
  • \$\begingroup\$ @asim normally you would bend the wires/coils a little bit to get a manual electrical balance and then, once the coils have been deformed and inevitably sprung back a little, the final balancing can be done with a small piece of ferrite. \$\endgroup\$ Commented Feb 25, 2022 at 14:12
  • \$\begingroup\$ After calculating CM, DM, Vs+ and Vs- for desired output range and to avoid min/max Vin limitations on InAmp, built the circuit but before I could attach it, this happened :electronics.stackexchange.com/questions/610356/… @Andy \$\endgroup\$ Commented Feb 28, 2022 at 23:19

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