Optimization of chopper amplifiers for speed and gain

Chopping for over 50 MHz gain‐bandwidth product current sense amplifiers achieving input noise level of 8.5 nV/√Hz

“…Starting in the 1970s and until the 1990s, several important works were published that utilized and extended the chopping principle to achieve very low input offsets and to decrease the input-referred noise. 5,8,9 One popular example is the work of Enz et al, 10 where an input offset of just 5 μV and a noise floor of approximately 63 nV/√Hz were achieved at a power consumption of only 34 μW, which are remarkable results for the used 3 μm technology. At that time, the authors even went as far as claiming that the amplifier is free of 1/f-noise.…”

“…In Table 1, a brief overview over typical specifications for the above‐mentioned application is given regarding bandwidth (BW), gain, input impedance (| Z in |), input offset voltage V OS,in , and input noise as root mean square value V noisein,rms , and power spectral density (PSD), for two cases: (1) small shunt resistors (mΩ range) and high bandwidth (input‐signal frequency above 200 kHz) and (2) very small shunt resistors (sub‐mΩ range) and very high bandwidth (input‐signal frequency above 1 MHz). In literature, however, the concept of chopping is typically used for a strong reduction of the input‐referred noise and offset at the costs of a remaining ripple on the output signal, which has to be dampened by low‐pass filters that also affect the bandwidth 3–5 . For the proposed application, one could expect a significant improvement when using a chopper‐based CSA, but the bandwidth limitation may hinder the applicability, since typical choppers are running at kHz clocks with a usable signal bandwidth that is significantly lower, as shown in the next paragraph.…”

“…In literature, however, the concept of chopping is typically used for a strong reduction of the input-referred noise and offset at the costs of a remaining ripple on the output signal, which has to be dampened by low-pass filters that also affect the bandwidth. [3][4][5] For the proposed application, one could expect a significant improvement when using a chopper-based CSA, but the bandwidth limitation may hinder the applicability, since typical choppers are running at kHz clocks with a usable signal bandwidth that is significantly lower, as shown in the next paragraph. Compared with prior works, we utilize a higher chopping frequency in order to fulfill the tough bandwidth requirements, which is challenging on the one hand and leads to some drawbacks, which will be discussed below.…”