[SOLVED] Shunt resistor current meassurements - opamp design concept

[ikorman]

Hi there

I have built motor test stand for brush less motors for my RC equipment (testing RPM, thrust, etc.). One of the thing I would like to add to my system is motor current measurements capability. I have chose to use shunt resistor which for 100A gives 75mV, which I will connect to my Arduino board and use ADC to sample shunt voltage and calculate the current.

My problem is how to design quality opamp circuit that will bring voltages from 0-75mv range to 0-5V. As I want to measure currents with precision of at least 0.1 A, that means that I need to able do measure voltages from 0.1 mV or less. I'm not so good in pure analog electronics (I'm more digital/uC oriented guy ;-)), but I'm aware of of problems like rail-to-rail inputs, offset voltage, inputs near zero, noise and similar things regarding opAmps. I would like to use something like AD620 or similar (precise, high gain, very low noise, etc.)

What would you recommend for me for this particular use case? Would AD620 work for this?

Many thx.
Ivan

 

[mikersia]

From your spec it is clear that 75mV signal have to be converted to code with resolution better 0.1mV,- that means at least 10 bit ADC conversion. For this application is better to skip intermediate amplifier and to use high resolution (i.e. 16bits or more) ADC with oversampling to average switching noise. Depending of oversampling ratio 10 lowest and averaged () bits should be used for postprocessing. Samples around motor control switching edges should be zeroed to reduce measurements error. This way will give you a robust and precision solution. By the way an input offset can be calibrated or rejected using chopper.

 

[keith1200rs]

I have to disagree with mikersia. Assuming you have an ADC on the Arduino (I know nothing about them) I would recommend using an amplifier. Something like the AD620 is not a bad choice although it is not rail to rail I/O. Depending on what power supplies you have available you may be better off looking for a rail-rail single supply 5V one - it will be easier to use. The AD623 is single supply, the input can work below ground and it has a rail to rail output. You may want to include some capacitors for removing noise - the circuit figure 43 for example. Larger capacitors may be a good idea as your interference may be fairly low frequency.

Another option is simply to use an opamp. It partly depends on how close the opamp will be to the shunt resistor and hence how much common mode noise you will get. Again, look for single supply, rail to rail output with an input range that extends to ground.

Keith.

 

[ikorman]

I have to disagree with mikersia. Assuming you have an ADC on the Arduino (I know nothing about them) I would recommend using an amplifier. Something like the AD620 is not a bad choice although it is not rail to rail I/O. Depending on what power supplies you have available you may be better off looking for a rail-rail single supply 5V one - it will be easier to use. The AD623 is single supply, the input can work below ground and it has a rail to rail output. You may want to include some capacitors for removing noise - the circuit figure 43 for example. Larger capacitors may be a good idea as your interference may be fairly low frequency.

Another option is simply to use an opamp. It partly depends on how close the opamp will be to the shunt resistor and hence how much common mode noise you will get. Again, look for single supply, rail to rail output with an input range that extends to ground.

Keith.

Thx. Will try AD623.

Ivan