Current Measurement for Low-Power Applications

For low-power applications, current consumption and measurement is very important to ensure the target board can run as expected. There are several tools to achieve this purpose. As a maker, I choose to design a current-to-voltage converter by using a shut-resistor and an OP-AMP.

Low-power applications usually consume few uA to tens of mA. So, I prefer the converter to be able to measure from few uA to 20mA without changing OP-AMP gain. TI INA225 seems good for my target. The gain is set to 100X and the shut-resistor is 10 ohm, so 1V is got per mA. This is good since the voltage reading on oscilloscope can be easily converted to mA.

In order for INA225 to output 20V (VOUT) for maximum 20mA load, the operating voltage of INA225 must be higher than 20V. For this, a boost converter is used to convert the input voltage (usually 3V) to 22V and then a LDO is used to regulate it to 20.4V. The power supply to OP-AMP, boost and LDO can be from input power source (VIN) or from an aux power source (VAUX). This feature is useful when a coin-cell battery is used. I don't want these circuit to affect the performance of coin-cell battery.

To test it. An ANT+ target board is connected to VOUT. The target will transmit RF signal at 4Hz. We can see the peak voltage is about 12.2V which means 12.2mA. And the period is about 250ms.

KNOWN ISSUE: Everything seems working fine. But I found there is an offset voltage output, about 14mV, when no load. I guess this is caused by the offset voltage of INA225. Mmmm... calibration seems required for few uA measurement. Well, at least this converter is still very useful for tens of uA to 20mA measurement.

When 20mA is drained through shut-resistor, the voltage drop is about 0.2V since the resistance is 10 ohm. So, for 3V battery, the actual voltage applied to target is only 2.8V. This should be fine since the internal resistance of battery will become larger than 10 ohm as time goes by.

Also notice there are three resistors (R7=100, R8=500 and R9=1K ohm) can be selected by jumper. This is used to simulate the internal resistance of battery. When out of battery, the internal resistance of a battery will become quite large, usually hundreds of ohms. With this feature, I can estimate the performance of the bulk capacitors on target board. The bulk capacitors are required to provide instant power for RF transmission. So, this feature let me easily adjust bulk capacitance.