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# INA169: Wrong output voltage

Part Number: INA169

I'm testing out a simple shunt resistor current sensor circuit based on the schematic provided in the datasheet. My Rs is 1mOhm and Rl is 10kOhm.

When passing a current of 0.50A (verified with another current sensor + ammeter), the voltage across the shunt resistor is 0.5mV but the voltage at Vo is 10mV (expected around 5mV), and when current is increased to 1A, the voltage across shunt resistor is 0.1mV but Vo is ~20mV (expected around 10mV).

I have checked the connections and tried connecting V+ to both an external 5V source and directly to VIN+ but I still get the same results. I have also swapped out to another INA169 (from the same batch) and obtained similar results. Is there anything that I might be missing or I have done wrongly?

• The INA160 has a specified offset voltage of up to 1 mV, so these errors are possible. (What is the output with 0 A?)

What is the maximum current? The datasheet suggests to use a shunt resistors that drops 100 mV for that.

• Hi Xiao,

At 0.5A through R shunt, the Vsense generated is only 0.5mV. Like Clemens said, the Max offset voltage of the device can be up to +/- 1mV, meaning that the input offset voltage could be twice as large as your signal of interest giving you large errors like this.

Your shunt resistor and the signal generated by it is too small for your application.

From the data sheet:

You should ideally select a shunt resistor value where the lowest current you will measure gives a Vsense (voltage across the shunt resistor) measurement of 50mV (your Vsense is 100 times smaller than this recommendation) and your largest current reading corresponds to 100mV Vsense (it can be higher, up to 500mV for accurate measurements, but this range corresponds to the best performance).

This means if 0.5A is the smallest current you will measure, your shunt should be around 100mΩ, or about 0.2Ω if 0.5A is the highest current you intend to measure. The higher the shunt resistor value is the more accurate your reading, at a cost of higher power dissipation. In this case 1mΩ is too small to use for this application.

Make sure the power rating of the chosen shunt resistor is appropriate.

There may also be some leakage current somewhere in your circuit which contributes to additional error.

Best Regards,

Angel

• Hi Angel,

Thank you for the very detailed reply. I intend to use the current sensor for a range of 0-40A, with the shunt resistor being rated for 3W (1.6W max at 40A), however when testing it with a high current load (motor + ESC), I get the similar scaling errors of the Vout being ~2 times of what is expected even at high current (10-20A). Could there be other possible factors contributing to this issue? Is it also possible to design the circuit to have better accuracy at lower currents after taking into account the offset at 0 current?

Thank you very much!

• Hi Xiao,

If a larger shunt resistor value is not an option, I would suggest looking at a part that has a lower input offset voltage spec to achieve better accuracy at lower currents. You can look at our catalog on this link and filter out parts with your required specs: Current-sense amplifiers product selection | TI.com

We also have an error tool that shows you percentage error at different currents based on your setup conditions and allows you to compare performance for different devices: https://www.ti.com/tool/download/CS-AMPLIFIER-ERROR-TOOL

Have you tried using an oscilloscope to observe the output of the amplifier? There may be some transients/stability issues causing error that can't be observed by the ammeter/DMM reading.

Best,

Angel