2 Internal Operation of Choppers

From a very high level, a traditional op-amp internal topology has at least two stages: transconductance, and transimpedance. The transconductance stage amplifies the differential input signal and converts the signal to a current (V_in × g_m = i_out). The output current of the transconductance stage drives the transimpedance stage which converts the transconductance current to a voltage. The feedback element in the transimpedance stage is the Miller capacitance (C_c), and the transconductance output current charges this capacitor to convert the current to a voltage. The chopper amplifier has the same two stages as the traditional amplifier, but the input and output of the transconductance stage is connected through a switch matrix that reverses the polarity of the input and output connections during each chopping calibration cycle (see Figure 2-1). The rest of this section documents how this switching of the chopper allows for normal amplifier amplification while minimizing offset.

Figure 2-2 illustrates the relationship between the input and output for each half of the chopping calibration cycle. In the first half of the calibration cycle, the input signal is inverted at the input of the transconductance amplifier. The output of this stage is simultaneously inverted before the output is applied to the transimpedance stage Miller capacitance. During the second half of the calibration cycle neither the input or output are inverted. Thus, in each half of the calibration cycle, the signal driving the transimpedance stage Miller capacitance is the same, and the amplifier is effectively the identical to the traditional op amp from a signal phase perspective.

Figure 2-3 illustrates the relationship between the input offset voltage and output for each half of the chopping calibration cycle. Notice that the polarity of the input offset on the input of the transconductance (g_m) stage is the same for both cycles. The resulting V_os signal at the output of the transconductance stage is flipped each half-cycle so that the Miller capacitor is charged oppositely each half-cycle. The transconductance stage converts the constant input offset voltage to a constant current. When a constant current is applied to a capacitor, the voltage on the capacitor changes linearly. Thus, the transconductance stage output current generated by the input offset voltage causes the voltage on the Miller capacitor (C_c) to ramp linearly. The ramp signal changes direction each half-cycle, so that the offset voltage is translated into a triangle wave across the Miller capacitor (see Figure 2-4). The triangular waveform multiplied by a gain factor becomes the output offset of the amplifier. Since the average is close to zero, the average input offset voltage is approximately zero. Thus, in the case of an actual amplifier with inherent input voltage offset of ±1mV and offset drift of ±1μV/C, the addition of chopping scheme lowers both of them by the factor of 100, down to about ±10μV and ±10nV/C, respectively. Thus, the calibration achieved the goal of minimizing the average offset voltage; however, the triangle waveform is an undesirable signal at the amplifier output. For this reason, the triangular waveform is minimized through the use of a synchronous notch filter.

The Fourier theorem states that any periodic waveform can be expressed as a series of sinusoidal waveforms called a Fourier series. The different sinusoids in the series are called harmonics and appear at multiples of the fundamental frequency. Figure 2-4 shows the Fourier series for a triangular waveform. Triangle waveforms only have harmonics at odd multiples of the fundamental frequency. In this example the chopper calibration frequency is 100kHz so the triangle fundamental frequency is also 100kHz. Figure 2-4 also shows the synchronous notch filter frequency response. This filter is implemented with a switched capacitor circuit that synchronously integrates the chopped offset signal. Since the filter is synchronized to the chopping frequency, the filter has deep attenuation notches at all the harmonics of the triangle wave. Typical attenuation of the triangular offset ripple is on the order of 500 ×.

Figure 2-5 shows a more comprehensive chopper block diagram that includes the synchronous notch filter and a high-frequency path. The DC precision is determined by the transconductance input stage. At low frequencies, the high-frequency path has a much lower gain than the DC precision path, so the precision path controls the output. At higher frequencies, the gain of the high-frequency path dominates allowing for proper rolloff and phase margin.