Current Analyzer

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

The differential-pair toggle buttons are on the Inputs tab; the per-channel Offset, Scale, Units, the Current Measuring Resistor, and the Battery Capacity live in the Settings tab's Analog Channel Settings section. There is no separate pop-up dialog.

1. Open the Inputs tab and click CH3+/4- to put the CH3/CH4 input pair into differential mode (on Active-Pro Ultra the second pair button CH1+/2- is also available). The buttons are toggles, click again to disable that pair.
2. In the Settings tab's Analog Channel Settings section, enter the per-channel Offset (default 0.0) and Scale (default 1.0) for the differential pair. The application multiplies the measured voltage by Scale, then adds Offset, before displaying. To convert a shunt voltage drop to milliamps, use Scale = 1000 / R_ohms.
3. Set the Units text (default V) to the unit you want shown in the waveform name column, for example mA or A.
4. Enter the shunt resistor value in ohms into the Current Measuring Resistor (Ohms) field in the same Settings-tab section. This drives current calculations elsewhere in the app (battery-life estimation, for example).
5. Optionally, enter the battery capacity in the Battery Capacity (mHr) field next to it for battery-life estimation. (The on-screen label reads "mHr" but the value is treated as milliamp-hours.)

Edits are applied as you type, there is no Save button. The waveform display updates immediately.

Use the differential pair for current measurement to avoid ground reference errors when the shunt is not at the circuit ground potential, for example when measuring a high-side supply current.

Trigger on current events: Once an analog channel is displaying calibrated current, you can trigger on threshold crossings in the Buffer & Triggers tab, useful for catching sudden current spikes or drops during firmware events.

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 the Battery Capacity (mHr) field in the Settings tab's Analog Channel Settings section. The field label reads mHr but the value is interpreted as milliamp-hours. The application uses the measured current waveform plus this capacity to display an estimated remaining battery life.

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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.

Find rare power-spike events: Configure a falling-edge or rising-edge analog trigger on the current channel in the Buffer & Triggers tab with a generous post-trigger window. Let the system run; the application will isolate the moment the spike occurs and snap the view to it.