As indicated in Figure 59, the output of the integrated differential amplifier is internally biased to a common-mode voltage of 1.9 V. This voltage results in a bias current IIB through the resistive network R4 and R5 (or R4' and R5') used for setting the gain of the amplifier. The value range of this current is specified in theElectrical Characteristics table. This bias current generates additional offset error that depends on the value of the resistor R3. The initial system offset calibration does not minimize this effect because the value of the bias current depends on the actual common-mode amplitude of the input signal (as illustrated in Figure 60). Therefore, in systems with high accuracy requirements, a series resistor is recommended to be used at the negative input (AINN) of the AMC1306 with a value equal to the shunt resistor R3 (that is, R3' = R3 in Figure 59) to eliminate the effect of the bias current.
This additional series resistor (R3') influences the gain error of the circuit. The effect can be calculated using Equation 5 with R5 = R5' = 50 kΩ and R4 = R4' = 2.5 kΩ (for the AMC1306x05) or 12.5 kΩ (for the AMC1306x25).