Shunt resistor and measurement resolution

[engr_joni_ee]

I am using INA3221 bus and shunt monitor.

The following info is taken from datasheet.
Shunt Voltage (Vshunt) = -163 mV to +163 mV
1x LSB Shunt Voltage = 40 uV
1x LSB Bus Voltage = 8 mV
Differential Nonlinearity = +/- 0.1 x LSB
Shunt Voltage Gain Error = 0.1 % to 0.25 %
Bus Voltage Gain Error = 0.1 % to 0.25 %

I have two current requirement mentioned below. I need to measure currents and bus voltages using INA3221 which is triple channel bus and shunt monitor.

Requirement 1: Load voltage 2.75 V and load current 3.6 A.
Shunt Resistor = 39 mOhm
Shunt Voltage = Shunt Resistor x Current = 39 mOhm x 3.6 A = 140 mV
Power of Shunt Resistor = Shunt Voltage x Current = 140 mV x 3.6 A = 0.5 Watt
Current Resolution = ADC Voltage Resolution / Shunt Resistor = 40 uV/bit / 39 mOhm = 1.02 mA/bit

Requirement 2: Load voltage 3.00 V and load current 10 mA.
Shunt Resistor = 14 Ohm
Shunt Voltage = Shunt Resistor x Current = 14 Ohm x 10 mA = 140 mV
Power of Shunt Resistor = Shunt Voltage x Current = 140 mV x 10 mA = 0.002 Watt
Current Resolution = ADC Voltage Resolution / Shunt Resistor = 40 uV/bit / 14 Ohm = 0.003 mA/bit

Kindly let me know if I am missing anything in current resolution.

 

[KlausST]

Hi,

I´m positively surprised by the calculations. Well done.

The math is correct so far. (without reading the datasheet)
Some comments:
* please mind that the voltage drop of the shunt reduces the voltage to the load: Example: 2.75V - 0.14V = 2.61V.
* mind that at Power_ON or at some pulsed loads the peak current my go higher than the average one. This may lead to overvoltage at the INA inputs as well as to higher power dissipation at the shunts.
* I´m not sure what your 3.6A rating is. Is it pure flat DC, sine peak, averaged pulses, something else? For the ADC input range you need to use PEAK values. For the power dissipation you need to use RMS values.
* mind to use 4 wire (Kelvin wiring) to the 39mOhms shunt.
* I ecommend to install anti aliasing low pass filters (unless the datasheet mentiones otherwise). This avoids measurement artefacts and also reduces noise.

Klaus

 

[danadakk]

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Regards, Dana.