Chopping: a technique for noise and offset reduction
From Chapter 3 we came to the conclusion that high accuracy and large dynamic range will cost power. We have considered there only white noise. The 1/f noise or flicker noise decreases further the dynamic range of analog circuits. In the following chapter different methods to reduce 1/f noise and offset are being discussed. Chopping is a technique for noise and offset reduction employed to boost at the same time the accuracy and the dynamic range of analog circuits without extra penalty in power.
In the introductory part we are considering different ways of reducing offset and 1/f noise with their advantages and disadvantages. As a modulation technique, chopping modulates in a different way white noise and 1/f noise of amplifiers. Therefore the difference between 1/f noise modulation and white noise modulation is being introduced with a comparison to sampling methods. As we will see, chopping is the only method which reduces 1/f noise and offset without modifying the baseband white noise like in the sampling case . Although, chopping is a low frequency technique, there are applications where bandwidths of the signals are in the MHz range. At this frequency only the residual offsets generated from charge injection and slewing of the input stages will limit at the upper part the chopping frequency. A method to use chopper modulation at high frequencies is introduced and a low-voltage, low-power, chopped transconductance amplifier for mixed analogue digital applications will be presented. This OTA is meant for high-end audio applications. Chopping and dynamic element matching allow low noise and low residual offsets up to 1MHz. The sensitivity to substrate noise is tackled in the design.
In mixed level applications accurate voltage references are difficult to realize due to the lack of reproducible lateral pnp’s and the large offsets inherent to CMOS opamps. Another problem tackled in this chapter is related to the realization of a low power and accurate bandgap voltage reference in CMOS. It is shown that by using chopping techniques and a chopped OTA, the accuracy of a bandgap voltage reference can be improved about ten times without laser trimming and with the benefit of reducing the 1/f noise of the reference. The same chopped OTA for high-end audio applications has a power consumption of 600mW while in the bandgap example the power consumption is 7.5 mW. The two examples show that the term low power has to be related to the specific application and its own specs.