3 Chopping Input Current Transients

The input switches on chopper amplifiers are MOSFET transistors. Depending on the op-amp design either an N-channel metal-oxide semiconductor (NMOS) or a complementary metal-oxide semiconductor (CMOS) transistor switch is used. This section provides a short theoretical background of NMOS transistor operation, before covering the concept of charge injection and clock feedthrough. Charge injection and clock feedthrough are the phenomena that generate the transient current pulse at the switch input.

To turn on an NMOS switch, the gate-to-source is driven to a voltage greater than the threshold voltage. Driving the gate-to-source positively places the transistor into the linear region where the drain-to-source on-resistance is very low. To turn off the NMOS switch, the gate-to-source voltage is driven to zero to cut off the transistor so that the transistor effectively acts as an open. From a semiconductor physics perspective, driving the gate-to-source to a voltage less than the threshold voltage allows a depletion layer to form in the channel which means that the drain-to-source impedance is very high and only small leakage currents will flow (see Figure 3-1). Increasing the gate-to-source voltage above the threshold draws electrons from the drain and source into the channel creating a conductive inversion layer (see Figure 3-2).

Unfortunately, when the NMOS transistor is turned on and off, there are transient events called clock feedthrough and charge injection that generate a brief pulse of current at the input of the switch (transistor source). When the transistor turns on, the charge flows from the source and drains into the channel creating the inversion layer. When the transistor turns off, the charge in the inversion layer must return to the source and drain (see Figure 3-3). This transfer of charge, when the transistor turns on or off, is called charge injection. Clock feedthrough is the coupling of the clock signal through the parasitic gate-to-source capacitance. Both mechanisms generate a very brief but high amplitude current transient on the input of the NMOS switch.

Figure 3-4 shows the switched input of a chopper amplifier with NMOS transistor switches. Notice that the clocking signal for Q1 and Q2 is inverted on Q3 and Q4. Thus, when Q1 and Q2 are turned on, Q3 and Q4 are off and vise versa. Each time the clocking signal transitions, charge injection and clock feedthrough introduces a transient current at the inputs.

Figure 3-5 shows the measured transient current on OPA188. The bias current is defined as the average current flowing into the inputs of the amplifier. In this example the bias current is typically 160pA and is very small relative to the transients. The measured transients are 2μA in this example which is 12,500 times the magnitude of the bias current. The two large transients correspond to transitioning of the calibration circuit input switches. The smaller transient between the large transients occurs when the synchronous notch filter is switched. The notch filter is designed to switch half-way through the calibration cycle. The period of the calibration cycle for this example is 1.54μs (f_chop = 650kHz). The measured noise density of this device shows noise tones at the chopping frequency and the harmonics. The notch filter switching time is between the main chopping transition points. Thus, there is also a noise tone in the middle between the clocks due to the small transient from the notch filter (see Figure 3-6). Since these transients are currents, the impact of the transients can be minimized by reducing the source and feedback impedance.