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Current Measurement with DRV8303

myo_motor
Prodigy 250 points
Other Parts Discussed in Thread: DRV8303

Hello,

Using the DRV8303, we are trying to measure the current across a low-side shunt at 1 mOhm.  The connection is the same as RS1 on page 21.  We have the REF pin at 3.3 Volts.  In all cases I am measuring current with the sense amp at 10 V/V Gain.  I have two questions related to measuring the current in this way.

1.  Equation (1) on page 12 does not include the offset voltage that you measure when you use the DC_CAL functionality.  If I set EN_GATE low, I find the voltage is 0.3 Volts (0.306 Volts as an example reading).  If I keep EN_GATE high, the voltage coming out of SO1 is close to 1.65 Volts (1.651209 Volts as an example reading).  Which value should be used, and how should it be used in the equation?

2.  Disregarding the offset, I use Equation (1) but the readings don't appear to be correct.  I have a fixed load of 14.52 Ohms across two phases, and I'm sending PWM through the phases at 50%.  The multimeter measures a voltage of 13.8 Volts, so I would expect to see a measurement that yields about 1 A with Equation (1).  The voltage coming out of the sense amp in this case is 1.632 Volts, so the current is calculated as 181 mA.  What am I doing wrong in this case?

  • 0
    TI__Guru** 114905 points

    Hi Jared,

    This post was answered in ( LINK )

    As Nick pointed out, the current works out to 1.9209 A. The voltage provided by the multimeter is an average voltage. Assuming you are measuring the SO1 with the multimeter also, the voltage can be different depending on the load (resistive vs capacitive). 

    The best method to determine the current is a current probe or measure the voltage across the resistor with a voltage probe.

  • 0 in reply to Rick Duncan
    Prodigy 250 points

    I'm not sure I'm convinced the current could be so high, but I'm understanding now that the voltage of the sense amplifier needs to be sampled in a time-sensitive manner.  An instantaneous measurement doesn't mean much when using PWM control, correct?

    Does TI provide any literature on best methods to measure current using the DRV8303?

    I went ahead and swapped out the shunt resistor to 10 mOhm.  I connected a passive probe across the 10 mOhm shunt.  What I see  on the scope is quite interesting...there are period events that are triggering around 1 Volt:

    That makes sense that they're periodic with our PWMs at 40 KHz, but looking closer:

    That would mean 100 A!  

  • 0 in reply to myo_motor
    TI__Guru** 114905 points

    Hi Jared,

    This appears to be noise. The current should not be able to increase so quickly in an inductor.

    Do you have the ground of the scope at the resistor ground? Grounds can bounce causing these types of pulses. This is also the reason the current sense amplifier is differential.