5 Chopping Current Transient Impact on Offset Voltage

Figure 3-5 illustrates the amplifier transient input current due to the charge injection and clock feedthrough of the chopper input metal–oxide–semiconductor (MOS) switches. When these input transient currents flow through the feedback network and source impedance connected to the amplifier, the transients convert into voltage. The transients are very fast compared to the bandwidth of the op amp, so the transients do not fully settle. The average of the AC voltage signal from the chopping transients has a non-zero value. This average value acts as an additional input offset voltage that adds to the offset of the amplifier. Depending on the magnitude of the source and feedback impedance, the offset due to the chopping transients can be either negligible or can be significant compared to the offset of the amplifier. The specific resistance value where the transients begin to be significant depends on the bandwidth of the amplifier, chopping frequency, and transient I_B glitch magnitude.

Figure 5-1 shows a simple TINA-TI model for OPA188 that includes the transient current pulses (I_G1 and I_G2), bandwidth, and input capacitance. The current transient on the non-inverting input flows through the source impedance, and the transient on the inverting input flows through the impedance network. If these two transients are identical, and the feedback impedance matches the source impedance, then the transient input currents create offset voltages from the feedback and source impedance that cancel each other.

Figure 5-2 shows the response for the circuit shown in Figure 5-1. Since there is a slight mismatch in the current transients, the transients do not fully cancel and the output shows an offset error signal that varies with time. The DC average of the output offset signal is 25.8µV. Figure 5-3 shows the response for the same circuit with the feedback resistor shorted. Since R_FB is shorted, there is no canceling effect between the inverting and non-inverting signal. Thus, the average output offset increases to 204µV. Figure 5-4 shows the response for the same circuit with the source resistor shorted. Again, there is no canceling effect between R_FB and R_s since R_s is shorted (average offset is –187µV).

It is undesirable for the bias current transients of a chopper amplifier to produce offsets that are larger than the V_os of the device. The offset voltage generated by these current transients is dependent on the magnitude of the source impedance and the impedance of the feedback network. Figure 5-5 shows a graph of OPA388 offset voltage versus source impedance. The flat region on the left-hand side of the graph is where the inherent offset of the amplifier is dominant. The region where the curve increases with source impedance is where the current transients dominate the offset. In general, do not use the chopper amplifier with the larger source impedances where the transients dominate. The transition point where the source impedance is considered large is different for each chopper amplifier. Table 5-1 provides a list of chopper amplifiers and the associated maximum impedance to avoid the increase in offset voltage.