The theoretical output voltage of the circuit is V_OUT = [ (R_SENSE × R₃) / R₁ ] × I_CHARGE. In reality, however, as a result of the finite current gain of the transistor (β), the current that travels through R₃ is not I_CHARGE. Instead, R₃ is α × I_CHARGE or β / ( β+1) × I_CHARGE. A Darlington pair can be used to increase the β and performance of the measuring circuit.

Using the components shown in Figure 7-7 results in V_OUT ≈ 4000Ω × I_CHARGE. This result is needed to amplify a 1mA I_CHARGE to near full-scale of an analog-to-digital converter (ADC) with V_REF at 4.1V. A resistor, R₂ is used at the noninverting input of the amplifier, with the same value as R₁ to minimize offset voltage.

Selecting values per Figure 7-7 limits the current traveling through the R₁ – Q1 – R₃ leg of the circuit to under 1µA, which is on the same order as the LPV521 supply current. Increasing resistors R₁, R₂, and R₃ decreases the measuring circuit supply current and extends battery life.

Decreasing R_SENSE minimizes error due to resistor tolerance; however, this decrease also decreases V_SENSE = I_CHARGE × R_SENSE, and in turn, the amplifier offset voltage has a more significant contribution to the total error of the circuit. With the components shown in Figure 7-7, the measurement circuit supply current can be kept below 1.5µA and measure 100µA to 1mA.