8 Chopper Transient Noise

This section covers noise generated by the chopper input switching. Chopper switching noise is at the chopping calibration frequencies and multiples of this frequency. This noise results from the translation of transient current pulses into voltage through the source impedance and feedback network impedance. Thus, larger impedances increase the amplitude of this noise. The chopping noise shows up in the frequency domain as tones at specific frequencies, and in the time domain as regular transients (see Figure 8-1, and Figure 8-2 respectively).

The amplitude of the transient signals at the amplifiers output is relatively independent from closed loop gain. This is because the transient noise signal at the output is the bias current transient multiplied by the feedback resistor (V_O(chop noise) = I_{B_TRANS} x R_f). Conversely, the broadband noise at the output increases with gain, so the transient noise decreases relative to the broadband noise floor. Figure 8-3 shows the output noise density for OPA188 in different gain configurations. Notice how the broadband noise increases with gain but the transient noise tones are relatively constant for each gain case.

Noise and other error sources are normally referred to the input (RTI). Since the noise transients at the amplifier output are relatively independent of gain, the amplifier intrinsic noise referred to the input decreases by the gain factor (V_nRTI = V_nRTO/G). Figure 8-4 shows the measured noise for gain of 101V/V and 11V/V RTI. The important point is that relative to other error sources, the chopping noise error becomes less significant at higher gains.

The chopping transients can be minimized by using a simple RC filter on the amplifier output (see Figure 8-5). Although noise tones start at 650kHz for OPA188, much of the transient content is in the higher frequency harmonics. Thus, it is not necessary to use a very low frequency filter to minimize the transient noise signal. Figure 8-6 shows the noise without an external filter and with filters at 3.2MHz and 7.2MHz for the OPA188 in a gain of 101V/V. The 3.2MHz filter reduces the transient to be practically negligible compared to the broadband noise. Set the filter cutoff to less than 650kHz to minimize all of the transient harmonic content. In this example, the OPA188 closed loop bandwidth is 19.8kHz (BW = GBW/G = 2MHz/101 = 19.8kHz). Thus, adding an external RC filter with a cutoff less than the 650kHz chopping frequency does not impact the AC performance of the amplifier.