Miliohm meter circuit - precision issue

[ikorman]

Hi

I'm trying to design circuit for measurement of brushless motor winding resistance. Idea is that I use LM317 or similar as current source which will passed through motor windings. By measuring generated voltage, I will be able to calculate motor resistance.

I wanted to start with 100mA current. Typical motor winding resistance is around 0.1 ohm. That means that generated voltage would be around 10mV. As I'm using AVR micro controller as interface to my PC, I have to to amplify measured voltage to 0-5V range (for given example I would need G=500). ADC precision is 10 bits (1024 values).

Because I have single supply environment, I opted for instrumentation, rail-to-rail op amp AD623. My problem is following: maximum input offset voltage for that op amp is 200 uV. As I use G=500 does that mean that I could expect that my output could swing +/- 0.1V on output (2%)? If I measure motor with even lower resistance than 0.1 ohm, error would be higher. Am I considering this right?

I initially wanted to use 10mA current, but if above is correct error would be much worse. As I want best accuracy (that is, what is possible with 10 bit ADC), I see following possibilities to reduce that amplification error:

- increase measurement current to 1 A (problem with LM317 dissipation)
- use another op amp with lower Voff (can you recommend any other - single supply, rail to rail)

What would you recommend how to approach this issue?

Thx.
Ivan

 

[FvM]

Simply measure and compensate the amplifier offset voltage in digital processing. This will reduce the error to the maximum offset drift amount. Or perform an autozero with each measurement.

 

[ikorman]

Simply measure and compensate the amplifier offset voltage in digital processing. This will reduce the error to the maximum offset drift amount. Or perform an autozero with each measurement.

Is this one time action for single op amp or offset voltage will change over short period of times? How would you suggest to implement auto-zero function (e.g. short op amp inputs and measure output)?

Thx.
Ivan

 

[FvM]

Is this one time action for single op amp or offset voltage will change over short period of times?

The change is mainly caused by enviroment temperature variations. Refer to the datasheet for details.

(e.g. short op amp inputs and measure output)?

Yes, you can either use a relay or an analog switch.

 

[ikorman]

The change is mainly caused by enviroment temperature variations. Refer to the datasheet for details.

Yes, you can either use a relay or an analog switch.

What will happen in case that Voff is -200uV and I use op amp with rail to rail output? Should I not get in that case 0 at output and therefore could not measure the error?

Ivan

 

[FvM]

Good point. You should connect the AD623 reference input to a positive voltage to get an input range below zero.