ADC Current sensing using a Potential Divider

[Thomas Carrington]

Posted on January 20, 2018 at 16:48

Hi All,

I'm going to be building a motor control circuit, and what I would like is to be able to monitor the voltage and current going into the motor.

Was going to use the STM32F108's inbuild ADC to measure this, so I am limited to an input voltage of ~3.3Vs - the motor is going to be connected to a 12V @ ~2A power supply.

From a datasheet that I found on the web, I saw that I can use a potential divider to step down the voltage to within the ADCs input range - Datasheet on the LTC2991 p.25 - shows a circuit with a 0 to 40V & 0 to +/-2.2A power supply.

I understand how the calculation for the voltage resolution is done - its the inverse of the potential divider multiplied by the single point resolution.

I have

NO

idea how the current resolution is calculated and was hoping that someone within this community might be able to educate me on what I am missing in getting to the number stated on this sheet.

Regards,

  Thomas

[Andrew Neil]

Posted on January 20, 2018 at 19:10

The 

LTC2991

 is not an ST product - so the ST forum is not really the place to be asking for technical support on it!

You should contact Analog Devices (who recently acquired  Linear Tech) for support with their products...

[Andrew Neil]

Posted on January 20, 2018 at 19:13

 ,

 ,

http://www.linear.com/product/LTC2991 ♯ techsupport

 ,

[Thomas Carrington]

Posted on January 20, 2018 at 19:19

My bad ^^;

Was just using there datasheet as an example of what I had found on the web for down stepping the voltage/current to be within the sensing range of the desired ADC. Though I will get into contact with them and see if they can provide some help.

Cheers

[Andrew Neil · Accepted solution]

Posted on January 20, 2018 at 19:25

If you're using one of those, it does all the measurements for you, and you jut read the results over the I2C - so you wouldn't be using the STM32's ADC at all.

IF you want to use 

the STM32's ADC to measure current, you will  need a current sense amplifier (CSA) to give a usable voltage.

ST have a range of CSAs: 

http://www.st.com/en/amplifiers-and-comparators/current-sensing.html?querycriteria=productId=SC1264

  

[Andrew Neil]

Posted on January 20, 2018 at 19:28

You might also want to look at how the Nucleo Power Measurement shield does it:

http://www.st.com/en/evaluation-tools/x-nucleo-lpm01a.html

 

[Thomas Carrington]

Posted on January 20, 2018 at 19:45

Thanks for pointing me in the direction of those devices, will look into them (that Power shield is HUGE!)

[Andrew Neil]

Posted on January 20, 2018 at 20:42

not suggesting that you actually use the power shield for your motor - just that you could look at the techniques it uses for its measurements ...