One of the methods of measuring a current is using a coulombmeter. A coulombmeter consists of a capacitor within a negative feedback loop. The current is determined by the change in charge on the capacitor over time.

In Coulomb’s law, charge Q is determined by the known value of capacitance C and measured voltage V across the capacitor. See Equation 1.

where:

• C is the capacitance in farads of the integration capacitor
• V is the potential in volts across the integration capacitor
• Q is the charge in coulombs on the integration capacitor

Figure 2-1 Basic Coulumbmeter Schematic

However, the flow of individual charges is not uniform due to noise and other random variables. One of the advantages of a coulombmeter is that the random flow for a certain time interval is averaged. The resolution of the measurement can be improved as longer integration times are allowed.

Current flow into the inverting terminal of the coulombmeter can create a voltage that is an integration function of the current versus time. Applying current flow into the inverting terminal of the amplifier and the circuit integrates the current over time. The op amp outputs voltage across the integration capacitor. Measuring the change in voltage over a specific time multiplied by capacitance gives current, measured in amperes.

Figure 2-2 Change in Output Voltage versus Time

Where:

• V is the voltage across the capacitor in volts
• t is the time in seconds
• C is the capacitance in Farads
• I is the current in Amperes