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Current sense amp + op-amp buffer + ADC: Measuring down to 0 with single supply


Measuring Resistance of a Wire With an ADCSurge protection for ADC inputs measuring DC from batteries and switching supply?Interfacing Current shunt monitor with 16bit ADCHow does an ADC with an unequal split supply work?Signal buffer with op-amp clamping and voltage biasDAC based voltage supply (current buffered) with current monitoring: shorting/instability?How to calculate the value of a current sense resistor for use with an ADC?Output to ground rail on single-supply op-amp with grounded loadIs a buffer needed to measure the output of a power supply using an ADC?Single supply op amp with input attenuator













1












$begingroup$


I'm thinking about current sensing with a high dynamic range (10mA-20A) and using LTC6102 as a high-side current sense amp (the voltage would be 54.6V max, a 13S6P Li-ion battery).



The ADC I'm planning to use is LTC1407 (12bit 1.5Msps).



Planning to use OPA2365 as a unity-gain buffer between the current-sense amp and the ADC.



The current-sense amp provides an output current proportional to the sense voltage and given the high voltage and small package size the output current has to be 1mA full scale which requires a rather high value output resistor and thus a buffer is needed between the current-sense amp and the ADC.



The op-amp requires a small (-0.1V) negative supply for the output to go down to 0 and it's important to go down to 0 in my case because of the high dynamic range I want.



I could try and do a negative supply e.g. a crude one with an additional battery between ground and the negative supply of the op-amp but I would rather avoid it to simplify the circuit.



Is there a way that I can measure down to 0 without a negative supply voltage for the op-amp in this case?



I'm thinking of maybe putting a diode in series with the output resistor of the current-sense amp to offset the output voltage and then correct the scale of the ADC output accordingly, but I'm not sure if this will work. For low currents the diode would be in the region where small current changes would cause comparable voltage changes I suppose.










share|improve this question









$endgroup$







  • 3




    $begingroup$
    There are little charge pump IC intended to produce just a little negative voltage for things like this. From either Analog Devices, TI, or Linear. I don't remember.
    $endgroup$
    – Toor
    4 hours ago











  • $begingroup$
    No offsets will work, as going down to true zero volts is a function of the op-amps output stage. Some rail-to-rail op-amps can get down to within 100mV of zero, but it is very easy to create a negative voltage from a TLC555 timer and some 1N4148 diodes. You cannot have what you want without some type of compromise.
    $endgroup$
    – Sparky256
    4 hours ago










  • $begingroup$
    ti.com/product/TPS60403
    $endgroup$
    – Toor
    4 hours ago










  • $begingroup$
    You have selected a differential ADC, you could use a slightly elevated voltage as the OPA2365 reference and for the ADC negative input, like 0.2V.
    $endgroup$
    – pserra
    4 hours ago
















1












$begingroup$


I'm thinking about current sensing with a high dynamic range (10mA-20A) and using LTC6102 as a high-side current sense amp (the voltage would be 54.6V max, a 13S6P Li-ion battery).



The ADC I'm planning to use is LTC1407 (12bit 1.5Msps).



Planning to use OPA2365 as a unity-gain buffer between the current-sense amp and the ADC.



The current-sense amp provides an output current proportional to the sense voltage and given the high voltage and small package size the output current has to be 1mA full scale which requires a rather high value output resistor and thus a buffer is needed between the current-sense amp and the ADC.



The op-amp requires a small (-0.1V) negative supply for the output to go down to 0 and it's important to go down to 0 in my case because of the high dynamic range I want.



I could try and do a negative supply e.g. a crude one with an additional battery between ground and the negative supply of the op-amp but I would rather avoid it to simplify the circuit.



Is there a way that I can measure down to 0 without a negative supply voltage for the op-amp in this case?



I'm thinking of maybe putting a diode in series with the output resistor of the current-sense amp to offset the output voltage and then correct the scale of the ADC output accordingly, but I'm not sure if this will work. For low currents the diode would be in the region where small current changes would cause comparable voltage changes I suppose.










share|improve this question









$endgroup$







  • 3




    $begingroup$
    There are little charge pump IC intended to produce just a little negative voltage for things like this. From either Analog Devices, TI, or Linear. I don't remember.
    $endgroup$
    – Toor
    4 hours ago











  • $begingroup$
    No offsets will work, as going down to true zero volts is a function of the op-amps output stage. Some rail-to-rail op-amps can get down to within 100mV of zero, but it is very easy to create a negative voltage from a TLC555 timer and some 1N4148 diodes. You cannot have what you want without some type of compromise.
    $endgroup$
    – Sparky256
    4 hours ago










  • $begingroup$
    ti.com/product/TPS60403
    $endgroup$
    – Toor
    4 hours ago










  • $begingroup$
    You have selected a differential ADC, you could use a slightly elevated voltage as the OPA2365 reference and for the ADC negative input, like 0.2V.
    $endgroup$
    – pserra
    4 hours ago














1












1








1


1



$begingroup$


I'm thinking about current sensing with a high dynamic range (10mA-20A) and using LTC6102 as a high-side current sense amp (the voltage would be 54.6V max, a 13S6P Li-ion battery).



The ADC I'm planning to use is LTC1407 (12bit 1.5Msps).



Planning to use OPA2365 as a unity-gain buffer between the current-sense amp and the ADC.



The current-sense amp provides an output current proportional to the sense voltage and given the high voltage and small package size the output current has to be 1mA full scale which requires a rather high value output resistor and thus a buffer is needed between the current-sense amp and the ADC.



The op-amp requires a small (-0.1V) negative supply for the output to go down to 0 and it's important to go down to 0 in my case because of the high dynamic range I want.



I could try and do a negative supply e.g. a crude one with an additional battery between ground and the negative supply of the op-amp but I would rather avoid it to simplify the circuit.



Is there a way that I can measure down to 0 without a negative supply voltage for the op-amp in this case?



I'm thinking of maybe putting a diode in series with the output resistor of the current-sense amp to offset the output voltage and then correct the scale of the ADC output accordingly, but I'm not sure if this will work. For low currents the diode would be in the region where small current changes would cause comparable voltage changes I suppose.










share|improve this question









$endgroup$




I'm thinking about current sensing with a high dynamic range (10mA-20A) and using LTC6102 as a high-side current sense amp (the voltage would be 54.6V max, a 13S6P Li-ion battery).



The ADC I'm planning to use is LTC1407 (12bit 1.5Msps).



Planning to use OPA2365 as a unity-gain buffer between the current-sense amp and the ADC.



The current-sense amp provides an output current proportional to the sense voltage and given the high voltage and small package size the output current has to be 1mA full scale which requires a rather high value output resistor and thus a buffer is needed between the current-sense amp and the ADC.



The op-amp requires a small (-0.1V) negative supply for the output to go down to 0 and it's important to go down to 0 in my case because of the high dynamic range I want.



I could try and do a negative supply e.g. a crude one with an additional battery between ground and the negative supply of the op-amp but I would rather avoid it to simplify the circuit.



Is there a way that I can measure down to 0 without a negative supply voltage for the op-amp in this case?



I'm thinking of maybe putting a diode in series with the output resistor of the current-sense amp to offset the output voltage and then correct the scale of the ADC output accordingly, but I'm not sure if this will work. For low currents the diode would be in the region where small current changes would cause comparable voltage changes I suppose.







operational-amplifier adc current-measurement single-supply-op-amp






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 4 hours ago









axkaxk

372315




372315







  • 3




    $begingroup$
    There are little charge pump IC intended to produce just a little negative voltage for things like this. From either Analog Devices, TI, or Linear. I don't remember.
    $endgroup$
    – Toor
    4 hours ago











  • $begingroup$
    No offsets will work, as going down to true zero volts is a function of the op-amps output stage. Some rail-to-rail op-amps can get down to within 100mV of zero, but it is very easy to create a negative voltage from a TLC555 timer and some 1N4148 diodes. You cannot have what you want without some type of compromise.
    $endgroup$
    – Sparky256
    4 hours ago










  • $begingroup$
    ti.com/product/TPS60403
    $endgroup$
    – Toor
    4 hours ago










  • $begingroup$
    You have selected a differential ADC, you could use a slightly elevated voltage as the OPA2365 reference and for the ADC negative input, like 0.2V.
    $endgroup$
    – pserra
    4 hours ago













  • 3




    $begingroup$
    There are little charge pump IC intended to produce just a little negative voltage for things like this. From either Analog Devices, TI, or Linear. I don't remember.
    $endgroup$
    – Toor
    4 hours ago











  • $begingroup$
    No offsets will work, as going down to true zero volts is a function of the op-amps output stage. Some rail-to-rail op-amps can get down to within 100mV of zero, but it is very easy to create a negative voltage from a TLC555 timer and some 1N4148 diodes. You cannot have what you want without some type of compromise.
    $endgroup$
    – Sparky256
    4 hours ago










  • $begingroup$
    ti.com/product/TPS60403
    $endgroup$
    – Toor
    4 hours ago










  • $begingroup$
    You have selected a differential ADC, you could use a slightly elevated voltage as the OPA2365 reference and for the ADC negative input, like 0.2V.
    $endgroup$
    – pserra
    4 hours ago








3




3




$begingroup$
There are little charge pump IC intended to produce just a little negative voltage for things like this. From either Analog Devices, TI, or Linear. I don't remember.
$endgroup$
– Toor
4 hours ago





$begingroup$
There are little charge pump IC intended to produce just a little negative voltage for things like this. From either Analog Devices, TI, or Linear. I don't remember.
$endgroup$
– Toor
4 hours ago













$begingroup$
No offsets will work, as going down to true zero volts is a function of the op-amps output stage. Some rail-to-rail op-amps can get down to within 100mV of zero, but it is very easy to create a negative voltage from a TLC555 timer and some 1N4148 diodes. You cannot have what you want without some type of compromise.
$endgroup$
– Sparky256
4 hours ago




$begingroup$
No offsets will work, as going down to true zero volts is a function of the op-amps output stage. Some rail-to-rail op-amps can get down to within 100mV of zero, but it is very easy to create a negative voltage from a TLC555 timer and some 1N4148 diodes. You cannot have what you want without some type of compromise.
$endgroup$
– Sparky256
4 hours ago












$begingroup$
ti.com/product/TPS60403
$endgroup$
– Toor
4 hours ago




$begingroup$
ti.com/product/TPS60403
$endgroup$
– Toor
4 hours ago












$begingroup$
You have selected a differential ADC, you could use a slightly elevated voltage as the OPA2365 reference and for the ADC negative input, like 0.2V.
$endgroup$
– pserra
4 hours ago





$begingroup$
You have selected a differential ADC, you could use a slightly elevated voltage as the OPA2365 reference and for the ADC negative input, like 0.2V.
$endgroup$
– pserra
4 hours ago











3 Answers
3






active

oldest

votes


















4












$begingroup$

You can get a small negative voltage by using an LM7705 which produces -232mV nominal output voltage using a charge pump.



enter image description here



The advantage of using that part over a typical garden-variety inverting charge pump converter (eg. +5 to -5) or inverting boost converter is that the worst-case negative output voltage generally falls within the maximum negative input voltage of something like your ADC converter (-300mV in your case, which is typical), so you don't need to try to clamp the op-amp output/ADC input voltage near ground.



On the other hand, it's probably more expensive than some other solutions that would take more engineering effort, so this is just one of many possibilities.






share|improve this answer









$endgroup$












  • $begingroup$
    How do I estimate the impact of the switching noise of the charge pump on the performance of the op-amp? Should I look at the op-amp's PSRR?
    $endgroup$
    – axk
    3 hours ago










  • $begingroup$
    Output Voltage Ripple 4 mVpp 91kHz OPA365 60dB PSRR
    $endgroup$
    – Sunnyskyguy EE75
    2 hours ago











  • $begingroup$
    Yes, Tony gave you the parameters. Of course you can filter it further if you need to.
    $endgroup$
    – Spehro Pefhany
    2 hours ago






  • 1




    $begingroup$
    Package has external pins on 0.5mm centers, very easy to solder in many ways.
    $endgroup$
    – Sparky256
    2 hours ago


















1












$begingroup$

You could generate a small positive voltage, and use it as a virtual ground. Since you selected a differential ADC, its large common mode rejection can allow you to get away with a very simple way of generating that 0.2V reference voltage.





schematic





simulate this circuit – Schematic created using CircuitLab






share|improve this answer









$endgroup$




















    -1












    $begingroup$

    Others have given some tips, but you need to be aware that what you're trying to do a very iffy deal. The problem is that, effectively, you're trying to do





    schematic





    simulate this circuit – Schematic created using CircuitLab



    The problem with this is that it's not accurate. You see those grounds? Trust me, at 20 amps, they are not all at the same voltage. For the current levels you're talking about, stray resistance will be a big problem. It will depend critically on pc layout and system wiring. For that matter, since copper has a rather large resistance tempco, you may have problems with temperature sensitivity due to your stray resistances changing. If you have any other part of the system which draws much current, the issue can become even worse.



    Assuming that you are using a very small sense resistor, with small voltages produced in order to avoid large power dissipation in your resistor, I would really recommend a differential measurement, also called a Kelvin connection.





    schematic





    simulate this circuit



    where your amplifier is an instrumentation or differential amplifier.



    Trust me on this, single-ended current measurement, other than very crude limit sensing, is a recipe for heartbreak. Trying to do it with a single-supply amplifier only makes it worse.






    share|improve this answer









    $endgroup$












      Your Answer





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      3 Answers
      3






      active

      oldest

      votes








      3 Answers
      3






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      4












      $begingroup$

      You can get a small negative voltage by using an LM7705 which produces -232mV nominal output voltage using a charge pump.



      enter image description here



      The advantage of using that part over a typical garden-variety inverting charge pump converter (eg. +5 to -5) or inverting boost converter is that the worst-case negative output voltage generally falls within the maximum negative input voltage of something like your ADC converter (-300mV in your case, which is typical), so you don't need to try to clamp the op-amp output/ADC input voltage near ground.



      On the other hand, it's probably more expensive than some other solutions that would take more engineering effort, so this is just one of many possibilities.






      share|improve this answer









      $endgroup$












      • $begingroup$
        How do I estimate the impact of the switching noise of the charge pump on the performance of the op-amp? Should I look at the op-amp's PSRR?
        $endgroup$
        – axk
        3 hours ago










      • $begingroup$
        Output Voltage Ripple 4 mVpp 91kHz OPA365 60dB PSRR
        $endgroup$
        – Sunnyskyguy EE75
        2 hours ago











      • $begingroup$
        Yes, Tony gave you the parameters. Of course you can filter it further if you need to.
        $endgroup$
        – Spehro Pefhany
        2 hours ago






      • 1




        $begingroup$
        Package has external pins on 0.5mm centers, very easy to solder in many ways.
        $endgroup$
        – Sparky256
        2 hours ago















      4












      $begingroup$

      You can get a small negative voltage by using an LM7705 which produces -232mV nominal output voltage using a charge pump.



      enter image description here



      The advantage of using that part over a typical garden-variety inverting charge pump converter (eg. +5 to -5) or inverting boost converter is that the worst-case negative output voltage generally falls within the maximum negative input voltage of something like your ADC converter (-300mV in your case, which is typical), so you don't need to try to clamp the op-amp output/ADC input voltage near ground.



      On the other hand, it's probably more expensive than some other solutions that would take more engineering effort, so this is just one of many possibilities.






      share|improve this answer









      $endgroup$












      • $begingroup$
        How do I estimate the impact of the switching noise of the charge pump on the performance of the op-amp? Should I look at the op-amp's PSRR?
        $endgroup$
        – axk
        3 hours ago










      • $begingroup$
        Output Voltage Ripple 4 mVpp 91kHz OPA365 60dB PSRR
        $endgroup$
        – Sunnyskyguy EE75
        2 hours ago











      • $begingroup$
        Yes, Tony gave you the parameters. Of course you can filter it further if you need to.
        $endgroup$
        – Spehro Pefhany
        2 hours ago






      • 1




        $begingroup$
        Package has external pins on 0.5mm centers, very easy to solder in many ways.
        $endgroup$
        – Sparky256
        2 hours ago













      4












      4








      4





      $begingroup$

      You can get a small negative voltage by using an LM7705 which produces -232mV nominal output voltage using a charge pump.



      enter image description here



      The advantage of using that part over a typical garden-variety inverting charge pump converter (eg. +5 to -5) or inverting boost converter is that the worst-case negative output voltage generally falls within the maximum negative input voltage of something like your ADC converter (-300mV in your case, which is typical), so you don't need to try to clamp the op-amp output/ADC input voltage near ground.



      On the other hand, it's probably more expensive than some other solutions that would take more engineering effort, so this is just one of many possibilities.






      share|improve this answer









      $endgroup$



      You can get a small negative voltage by using an LM7705 which produces -232mV nominal output voltage using a charge pump.



      enter image description here



      The advantage of using that part over a typical garden-variety inverting charge pump converter (eg. +5 to -5) or inverting boost converter is that the worst-case negative output voltage generally falls within the maximum negative input voltage of something like your ADC converter (-300mV in your case, which is typical), so you don't need to try to clamp the op-amp output/ADC input voltage near ground.



      On the other hand, it's probably more expensive than some other solutions that would take more engineering effort, so this is just one of many possibilities.







      share|improve this answer












      share|improve this answer



      share|improve this answer










      answered 4 hours ago









      Spehro PefhanySpehro Pefhany

      210k5160423




      210k5160423











      • $begingroup$
        How do I estimate the impact of the switching noise of the charge pump on the performance of the op-amp? Should I look at the op-amp's PSRR?
        $endgroup$
        – axk
        3 hours ago










      • $begingroup$
        Output Voltage Ripple 4 mVpp 91kHz OPA365 60dB PSRR
        $endgroup$
        – Sunnyskyguy EE75
        2 hours ago











      • $begingroup$
        Yes, Tony gave you the parameters. Of course you can filter it further if you need to.
        $endgroup$
        – Spehro Pefhany
        2 hours ago






      • 1




        $begingroup$
        Package has external pins on 0.5mm centers, very easy to solder in many ways.
        $endgroup$
        – Sparky256
        2 hours ago
















      • $begingroup$
        How do I estimate the impact of the switching noise of the charge pump on the performance of the op-amp? Should I look at the op-amp's PSRR?
        $endgroup$
        – axk
        3 hours ago










      • $begingroup$
        Output Voltage Ripple 4 mVpp 91kHz OPA365 60dB PSRR
        $endgroup$
        – Sunnyskyguy EE75
        2 hours ago











      • $begingroup$
        Yes, Tony gave you the parameters. Of course you can filter it further if you need to.
        $endgroup$
        – Spehro Pefhany
        2 hours ago






      • 1




        $begingroup$
        Package has external pins on 0.5mm centers, very easy to solder in many ways.
        $endgroup$
        – Sparky256
        2 hours ago















      $begingroup$
      How do I estimate the impact of the switching noise of the charge pump on the performance of the op-amp? Should I look at the op-amp's PSRR?
      $endgroup$
      – axk
      3 hours ago




      $begingroup$
      How do I estimate the impact of the switching noise of the charge pump on the performance of the op-amp? Should I look at the op-amp's PSRR?
      $endgroup$
      – axk
      3 hours ago












      $begingroup$
      Output Voltage Ripple 4 mVpp 91kHz OPA365 60dB PSRR
      $endgroup$
      – Sunnyskyguy EE75
      2 hours ago





      $begingroup$
      Output Voltage Ripple 4 mVpp 91kHz OPA365 60dB PSRR
      $endgroup$
      – Sunnyskyguy EE75
      2 hours ago













      $begingroup$
      Yes, Tony gave you the parameters. Of course you can filter it further if you need to.
      $endgroup$
      – Spehro Pefhany
      2 hours ago




      $begingroup$
      Yes, Tony gave you the parameters. Of course you can filter it further if you need to.
      $endgroup$
      – Spehro Pefhany
      2 hours ago




      1




      1




      $begingroup$
      Package has external pins on 0.5mm centers, very easy to solder in many ways.
      $endgroup$
      – Sparky256
      2 hours ago




      $begingroup$
      Package has external pins on 0.5mm centers, very easy to solder in many ways.
      $endgroup$
      – Sparky256
      2 hours ago













      1












      $begingroup$

      You could generate a small positive voltage, and use it as a virtual ground. Since you selected a differential ADC, its large common mode rejection can allow you to get away with a very simple way of generating that 0.2V reference voltage.





      schematic





      simulate this circuit – Schematic created using CircuitLab






      share|improve this answer









      $endgroup$

















        1












        $begingroup$

        You could generate a small positive voltage, and use it as a virtual ground. Since you selected a differential ADC, its large common mode rejection can allow you to get away with a very simple way of generating that 0.2V reference voltage.





        schematic





        simulate this circuit – Schematic created using CircuitLab






        share|improve this answer









        $endgroup$















          1












          1








          1





          $begingroup$

          You could generate a small positive voltage, and use it as a virtual ground. Since you selected a differential ADC, its large common mode rejection can allow you to get away with a very simple way of generating that 0.2V reference voltage.





          schematic





          simulate this circuit – Schematic created using CircuitLab






          share|improve this answer









          $endgroup$



          You could generate a small positive voltage, and use it as a virtual ground. Since you selected a differential ADC, its large common mode rejection can allow you to get away with a very simple way of generating that 0.2V reference voltage.





          schematic





          simulate this circuit – Schematic created using CircuitLab







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 4 hours ago









          pserrapserra

          615313




          615313





















              -1












              $begingroup$

              Others have given some tips, but you need to be aware that what you're trying to do a very iffy deal. The problem is that, effectively, you're trying to do





              schematic





              simulate this circuit – Schematic created using CircuitLab



              The problem with this is that it's not accurate. You see those grounds? Trust me, at 20 amps, they are not all at the same voltage. For the current levels you're talking about, stray resistance will be a big problem. It will depend critically on pc layout and system wiring. For that matter, since copper has a rather large resistance tempco, you may have problems with temperature sensitivity due to your stray resistances changing. If you have any other part of the system which draws much current, the issue can become even worse.



              Assuming that you are using a very small sense resistor, with small voltages produced in order to avoid large power dissipation in your resistor, I would really recommend a differential measurement, also called a Kelvin connection.





              schematic





              simulate this circuit



              where your amplifier is an instrumentation or differential amplifier.



              Trust me on this, single-ended current measurement, other than very crude limit sensing, is a recipe for heartbreak. Trying to do it with a single-supply amplifier only makes it worse.






              share|improve this answer









              $endgroup$

















                -1












                $begingroup$

                Others have given some tips, but you need to be aware that what you're trying to do a very iffy deal. The problem is that, effectively, you're trying to do





                schematic





                simulate this circuit – Schematic created using CircuitLab



                The problem with this is that it's not accurate. You see those grounds? Trust me, at 20 amps, they are not all at the same voltage. For the current levels you're talking about, stray resistance will be a big problem. It will depend critically on pc layout and system wiring. For that matter, since copper has a rather large resistance tempco, you may have problems with temperature sensitivity due to your stray resistances changing. If you have any other part of the system which draws much current, the issue can become even worse.



                Assuming that you are using a very small sense resistor, with small voltages produced in order to avoid large power dissipation in your resistor, I would really recommend a differential measurement, also called a Kelvin connection.





                schematic





                simulate this circuit



                where your amplifier is an instrumentation or differential amplifier.



                Trust me on this, single-ended current measurement, other than very crude limit sensing, is a recipe for heartbreak. Trying to do it with a single-supply amplifier only makes it worse.






                share|improve this answer









                $endgroup$















                  -1












                  -1








                  -1





                  $begingroup$

                  Others have given some tips, but you need to be aware that what you're trying to do a very iffy deal. The problem is that, effectively, you're trying to do





                  schematic





                  simulate this circuit – Schematic created using CircuitLab



                  The problem with this is that it's not accurate. You see those grounds? Trust me, at 20 amps, they are not all at the same voltage. For the current levels you're talking about, stray resistance will be a big problem. It will depend critically on pc layout and system wiring. For that matter, since copper has a rather large resistance tempco, you may have problems with temperature sensitivity due to your stray resistances changing. If you have any other part of the system which draws much current, the issue can become even worse.



                  Assuming that you are using a very small sense resistor, with small voltages produced in order to avoid large power dissipation in your resistor, I would really recommend a differential measurement, also called a Kelvin connection.





                  schematic





                  simulate this circuit



                  where your amplifier is an instrumentation or differential amplifier.



                  Trust me on this, single-ended current measurement, other than very crude limit sensing, is a recipe for heartbreak. Trying to do it with a single-supply amplifier only makes it worse.






                  share|improve this answer









                  $endgroup$



                  Others have given some tips, but you need to be aware that what you're trying to do a very iffy deal. The problem is that, effectively, you're trying to do





                  schematic





                  simulate this circuit – Schematic created using CircuitLab



                  The problem with this is that it's not accurate. You see those grounds? Trust me, at 20 amps, they are not all at the same voltage. For the current levels you're talking about, stray resistance will be a big problem. It will depend critically on pc layout and system wiring. For that matter, since copper has a rather large resistance tempco, you may have problems with temperature sensitivity due to your stray resistances changing. If you have any other part of the system which draws much current, the issue can become even worse.



                  Assuming that you are using a very small sense resistor, with small voltages produced in order to avoid large power dissipation in your resistor, I would really recommend a differential measurement, also called a Kelvin connection.





                  schematic





                  simulate this circuit



                  where your amplifier is an instrumentation or differential amplifier.



                  Trust me on this, single-ended current measurement, other than very crude limit sensing, is a recipe for heartbreak. Trying to do it with a single-supply amplifier only makes it worse.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 1 hour ago









                  WhatRoughBeastWhatRoughBeast

                  50k22876




                  50k22876



























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