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I am using a INA4180-4181EVM to find a suitable solution for measuring 0-2A of current in a DC circuit. In the datasheet, there is no equation given for calculating the current based on the read output voltage. How can I calculate the current?

Datasheet

EDIT:

The maximum current that should be measured is 2A, the maximum heat dissipation is set to 1/3W. Here are my calculations based on the datasheet: $$R_{shunt} < \frac{PD_{max}}{I^2_{max}}$$ $$R_{shunt} < \frac{\frac{1}{3}W}{2^2A} = 0.0833\Omega $$ Meaning that the shunt resistor must not be larger than 0.0833 Ohms in order to satisfy the power dissipation requirement. To get a starting point for the gain selection, my calculations are as follows: $$ I_{max} \cdot R_{shunt} \cdot GAIN < V_{SP}\\ GAIN = \left(\frac{I_{max} \cdot R_{shunt}}{V_{SP}}\right)^{-1}\\ GAIN = \left(\frac{2A \cdot 0.075\Omega}{3.3V}\right)^{-1} = 22$$ Selecting a gain of 20 (and my ADC can read 0-3.3V): $$R_{shunt} = \left(\frac{2A \cdot 20}{3.3V}\right)^{-1} = 0.0825\Omega$$

However, (my understanding is that) by using this value the current is limited to exactly 2A, so it is better to select a value lower than 0.0825 to provide some headroom. I want to test the following values for my shunt resistor to find which one suits my circuit best: 0.070, 0.075 and 0.080.

The current calculation is then done as follows (using Rshunt = 0.08):

float Vshunt = V/20; // Gain reduction, V
float Rshunt = 0.08; // Ohms
float I = Vshunt/Rshunt; // A

Using the EVM board, this seems to match my Fluke multimeter pretty close (have not calculated exact difference yet since I dont have the correct Rshunt value at hand).

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1 Answer 1

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schematic

simulate this circuit – Schematic created using CircuitLab

This circuit technically measures a voltage differential, rather than a current. Each pair of inputs goes into an amplifier with a gain, and so the voltage you read (OUT) is the result of the difference in voltage between IN+ and IN- multiplied by the gain of that device - looks like there are 20V/V, 50V/V, 100V/V, and 200V/V options.

Section 3.1 of the datasheet describes how to use this. You have to use an external shunt resistor, connecting IN+ to the positive terminal of the resistor and IN- to the negative side. The current you are trying to measure should be passed through that shunt resistor which should have a small value. Then, you can use the voltage you measure at OUT to calculate the voltage difference across the shunt resistor, which you can then plug into V=i*R to find i. I'm attaching a rough schematic to illustrate what this looks like, although your actual circuit will be different.

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  • \$\begingroup\$ Ok, so lets say I measure 0.95V, have a 1 Ohm shunt resistor and use an IC with 20V/V gain. Is my voltage actually 0.95/20 then? So that my current is I=0.0475/1? \$\endgroup\$
    – eidetech
    Jun 20, 2023 at 14:58
  • \$\begingroup\$ @eidetech Yep, Vshunt=Vmeasured/20, i=Vshunt/Rshunt. You may want to run some trial and error using a load tester or a known current to characterize tolerances/offsets, but the math is pretty much that simple. \$\endgroup\$
    – InBedded16
    Jun 20, 2023 at 18:13
  • \$\begingroup\$ Sweet, thanks. I will try out the chip tomorrow and see if I can get it right. \$\endgroup\$
    – eidetech
    Jun 20, 2023 at 19:10
  • \$\begingroup\$ I have updated the post with my findings now. Do they look reasonable to you? Would love some feedback. \$\endgroup\$
    – eidetech
    Jun 21, 2023 at 13:44

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