Measuring a subsystem
Power source and measures subsystem's current and voltage
Set up Otii hardware, Otii Arc Pro, or Otii Ace Pro to measure a subsystem current and voltage over a shunt resistor in addition to measuring the complete system. Note that the resistor can be part of the actual system or applied externally.
Products required
Hardware
Otii hardware: Otii Arc Pro, or Otii Ace Pro.
DUT
Shunt resistor – How to choose the resistor?
Banana connectors cables + jumper wires
Software
Otii hardware setup
Connect Otii hardware to your computer via USB. If needed, add a DC power supply via the DC plug.
Connect Otii hardware’s banana connector positive lead to DUT positive (+) battery connector/power connector.
Connect Otii hardware’s banana connector negative lead to DUT negative (-) battery connector/power connector (GND).
Connect Otii hardware's ADC+ and ADC- across the shunt resistor for current measurement; no additional ground is needed for this. Make sure to connect the shunt resistor side with the high side to ADC+ and the one with the low side to ADC-. Note that if negative currents are measured, the high side has a lower potential than the low side.
[Optional] To measure voltage, connect AGND to the DUT's ground and measure the voltage between ADC+ and AGND.
In case of connecting the DUT using the battery connector, remove the battery prior to connecting it to the Otii hardware.
Otii software setup
Open the Otii 3 Desktop App, then select "Create a new Otii Project."
In the left sidebar, find the CONTROL section. Within this section, add the Otii hardware identified by your computer. Just click the add button on the right side of the hardware identified.
Considering the DUT's power ranges, set the required voltage for the added Otii hardware in the voltage box located on the left side of the power supply button.
Under "General Settings," make sure:
"Power Box" option is selected.
Set an OC (overcurrent) protection for the DUT.
Set the value of the shunt resistor in the "ADC resistor" field. Check how to choose the resistor if you are not sure how to do it.
Under "Channels," select "ADC current" to measure the subsystem current over a shunt resistor. In the case of measuring voltage, the "ADC voltage" must be selected as well. If the system itself needs to be measured, it can also be measured; select the channels desired to be measured in addition to the ones previously selected.
[Optional] The previously selected channels will be listed under the MEASUREMENTS section. In this section, the sampling rate, up to 50 ksps, can be set. (Only possible with the Otii Ace Pro)
Click the record button in the upper left corner of the toolbar to start recording DUT measurements. Since your DUT is not powered on yet, only noise measurements will be observed at first.
Under the CONTROL section, turn on the DUT by clicking on the power button located right next to the desired voltage assigned. Once turned on, the DTU measurements are being recorded.
Now it's time to validate, analyze, and optimize your embedded system or IoT devices.
Keep in mind
🔋 The voltage over the shunt resistor is measured differentially. No extra signal ground is needed if you only want to measure the current.
🔋 If voltage measurement is required, it is measured between ADC+ and AGND. In this case, AGND needs to be connected to the ground of the DUT.
The maximum voltage range on ADC pins are:
0V to 5 V for Otii Arc Pro
-10V to 25V for Otii Ace Pro
How to choose the resistor
For Otii Arc Pro
The differential input voltage range for the Otii Arc Pro ADC inputs goes from -81.9175 mV to 81.92 mV. Based on these values, the absolute maximum (peak) current through the sensing resistor must be determined.
Remember that by using Ohm's Law (V = I x R | I = V / R | R = V / I), the relationship between voltage, current, and resistance in an electrical circuit can be determined.
Here's an example to illustrate how to choose the resistor:
Suppose the system has a maximum peak current of 200 mA. By calculating, 0.08192 V / 0.2 A, the resistor value is equal to 0.41 ohms. But, the closest standard resistor value is 0.39 ohms. Resulting in a measuring range of 0.08192 V / 0.39 ohms = 0.210 A.
In conclusion, a 0.39 ohm sensing resistor will result in a measurement range of +/- 0.210 A.
For Otii Ace Pro
Following the same example as above, let's do the same for now but for the Otii Ace Pro.
The differential input voltage range for the Otii Ace Pro ADC inputs goes from -102.4 mV to 102.4 mV. Based on these values, the absolute maximum (peak) current through the sensing resistor must be determined. Here's an example to illustrate how to choose the resistor:
Suppose the system has a maximum peak current of 200 mA. By calculating, 0.1024 V / 0.2 A, the resistor value is equal to 0.512 ohms. But, the closest standard resistor value is 0.51 ohms. Resulting in a measuring range of 0.1024 V / 0.51 ohms = 0.200 A.
In conclusion, a 0.51 ohm sensing resistor will result in a measurement range of +/- 0.200 A.
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