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I want to measure a low current (from 0 to 75mA) using the attached schemtaic for ACS712. The ACS712 sensitivity is 185mv and I want to use the max gain of 47 to input vout to a microcontroller(0-5vmax).

I want to make sure that this is how to correctly callibrate:

V=I x sensitivity x max gain so when I is 0 V is 0 when I is 75m V is 0.652125

Also I don't have any negative current so how do I connect the offset(R4)?

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  • \$\begingroup\$ the image is unreadable, even the larger version when clicked \$\endgroup\$ – Neil_UK Mar 3 '17 at 19:11
  • \$\begingroup\$ I hope this is clearer @Neil_UK \$\endgroup\$ – Nemo Mar 3 '17 at 19:16
  • \$\begingroup\$ Why not use the circuit in the application note Application 3? \$\endgroup\$ – skvery Mar 3 '17 at 19:21
  • \$\begingroup\$ I need to output the voltage on a microcontroller (5v max) so using the suggested difference amplifier (Vout=10.75-3.3v1)would give 10.75v when I=0. and 8.6v when I=75mA.@skvery \$\endgroup\$ – Nemo Mar 3 '17 at 19:42
  • \$\begingroup\$ Not quite that's for a large current input of 30A. The App Note does not support 75mA full scale. You want to create 66,670mV/A \$\endgroup\$ – Sunnyskyguy EE75 Mar 3 '17 at 23:04
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The preferred method for R4 is to use V/2 using a resistor network with tolerance of <0.05% for matched equal values. The absolute value accuracy is unimportant.

enter image description here

This would not work for you due to Hall sensor noise equiv of 100mApp with +/-30A range.

Instead use an R shunt. 1 Ohm
RT0805BRD071RL Digikey

schematic

simulate this circuit – Schematic created using CircuitLab

Rail-Rail type needed with Gnd input sensing.

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  • \$\begingroup\$ O.K lets say I use V/2 how do I callibrate this circuit ? @Tony Stewart \$\endgroup\$ – Nemo Mar 3 '17 at 20:06
  • \$\begingroup\$ This is used to convert a bidirectional single supply referenced signal from V/2 now changed to 0V for use with mono-polarity signals. So you need Rail to rail Op Amp with high R values and with null input expect 0 output, prefer. 0.001 to show its not saturated. Ensure you have the correct polarity. If small values are very important, I suggest offset to 0.1V for null input and then null out in software. \$\endgroup\$ – Sunnyskyguy EE75 Mar 3 '17 at 20:15
  • \$\begingroup\$ O.K but I still don't know when the current is 75mA for example what will be the output voltage. I want to get the relationship between the input current and the output voltage to the microcontroller. \$\endgroup\$ – Nemo Mar 3 '17 at 20:35
  • \$\begingroup\$ If you want 0-75mA to become 0-5V with 185 mV/A setting then Gain = 5V/(185mV*75mA) =360 But this may not be the best choice for accuracy. Simply use a 75mV shunt R using 1 Ohm. This would be more accurate as typical noise is 7mV @66mV/A or 7/66m=106mA of noise. You can sense ground side or Vcc side with suitable diff amp. with gain of 5V/75mV= 106.7 \$\endgroup\$ – Sunnyskyguy EE75 Mar 3 '17 at 20:53
  • \$\begingroup\$ O.K thanks a lot. So would you recommend a specific diff amp for the application ? \$\endgroup\$ – Nemo Mar 3 '17 at 21:55

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