How to interface a sensing resistor to ADC?

[seyyah]

how to interface to adc

Hi, i'm driving a dc motor at 20khz with pwm. I will read the current that is drawn by motor. For this purpose i will read the the voltage of the sensing resistor which is connected to the low side. What must i care about or how must i interface this to the adc of ucontroller to read the current accurately and preferably with one reading.

 

[flatulent]

Re: how to interface to adc

You can use an op amp with the four resistor differential configuration. Another option is to put the current sensing resistor in the ground side of the motor and use an op amp to raise the voltage to be 2/3 of full scale on your adc.

 

[dannym]

Re: how to interface to adc

You can use an op amp with the four resistor differential configuration. Another option is to put the current sensing resistor in the ground side of the motor and use an op amp to raise the voltage to be 2/3 of full scale on your adc.

You can also get an instrumentation amplifier, which can do a fully differential input by itself. That saves you from having to mount external resistors and deal with the offset errors from resistor tolerances.

 

[IanP]

Re: how to interface to adc

Guys,
His main problem is not tolerance of resistors, which can be in the range of 0.1%, but how to fast convert pulses from PWM into RMS value that does not flactuate with pulses frequency.
I think you should use an amplifier as, say, 2-stage integrator to achieve this RMS value.

 

[flatulent]

Re: how to interface to adc

I forgot to say that you synchronize the ADC s/h with the center of the 20k pulses closest to the time you want to take measurements. This will give you the average current during the pulse. Then in yours software multiply this by the duty factor of the pulses. (I am assuming that the pwm is controlled by your digital circuitry.) Otherwise follow the advice above to low pass filter (integrate) the pulsed current reading.

 

[seyyah]

Re: how to interface to adc

Guys,
His main problem is not tolerance of resistors, which can be in the range of 0.1%, but how to fast convert pulses from PWM into RMS value that does not flactuate with pulses frequency.
I think you should use an amplifier as, say, 2-stage integrator to achieve this RMS value.

Hmm i think this can be applied. Does two stage for greater accuracy? Is this a kind of active filter? If so how should i choose the components' values?

 

[seyyah]

Re: how to interface to adc

I also want to ask that if there is any single chips for this purpose?

 

[visualart]

how to interface to adc

You can connect it to an op amp with the four resistor differential configuration. PWM output control the voltage valid.