Current Analyzer
Available on: ACTIVE-PRO · ACTIVE-PRO Ultra
The ACTIVE-PRO can measure current in your circuit by using the differential analog inputs to measure the voltage drop across a small series resistor placed in the current path. The application converts the measured voltage to a current reading automatically.
How It Works
Place a small series resistor in the current path you want to monitor. Connect the differential analog inputs across the resistor (CH3/+ and CH4/- on ACTIVE-PRO; CH1+/CH2- or CH3+/CH4- on ACTIVE-PRO Ultra). The voltage drop across the resistor is proportional to the current flowing through it (V = I × R). The application measures this differential voltage and divides by the resistor value to display current directly.
This approach allows you to see current changes correlated with your firmware's debug output and bus traffic — on the same timeline, to the same timestamp resolution. This is especially useful in battery-powered systems where tightening up power management code directly affects reliability and runtime.
Setup
- Open the Inputs tab on the right-side panel.
- Click the Gear icon at the right of the Analog Settings list box to open the Analog Channel Settings dialog.
- Enter your shunt resistor value (in ohms) in the Current Measurement Resistor field near the bottom.
- Click Save.
The analog channel will now display the measured current through the resistor rather than voltage.
Use the differential inputs for current measurement to avoid ground reference errors when the shunt resistor is not at the circuit ground potential (for example, when measuring a high-side supply current).
Choosing a Shunt Resistor Value
The value of the resistor should be small — between 50 mΩ and 10 Ω. The voltage drop across the resistor needs to be large enough to get reasonable resolution, but small enough to not impact your circuit.
A reasonable rule of thumb is to choose a resistor value that causes a voltage drop of 0.1V at your maximum current:
R = V / I = 0.1V / I_max
So for a system that has a maximum current of 200mA, your resistor should be R = 0.1V / 0.2A = 500 mΩ.
| Max System Current | Suggested Series Resistor |
|---|---|
| 10 mA | 10 Ω |
| 50 mA | 2 Ω |
| 100 mA | 1 Ω |
| 500 mA | 200 mΩ |
| 1 A | 100 mΩ |
Battery Capacity Estimation
Enter your battery capacity in mAh in the Battery Capacity field of the Analog Channel Settings dialog. The application will use the measured current waveform to calculate and display an estimated remaining battery life.
Tips
Correlating current with firmware state: Because the current measurement appears on the same timeline as your Active Debug Port output, you can directly correlate power mode transitions with the source code that triggered them.
Testing power supply response: Use A0 (analog output) to step the supply voltage, and capture the current response on the differential input channel. This gives you the full picture of your power supply's behavior with one capture.