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. In the following chapter different methods to reduce 1/f noise and offset are being discussed. These methods are based on sampling and modulation and their advantages and disadvantages are reviewed. Chopping is the only method which reduces 1/f noise and offset without modifying or at least without increasing the baseband white noise. Although, chopping is a low frequency technique, there are applications where bandwidths of the signals are in the MHz range. Here, the residual offsets generated from charge injection will limit 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 has been presented. This OTA is meant for high-end 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. Experimental results show residual offsets of less than 370m V up to 1MHz chopping frequency. Second order effects like charge injection and residual offsets are discussed. By chopping, the S/N is improved with about 6dB which brings a factor 4 reduction in power.
In mixed level applications accurate voltage references are difficult to realize due to the lack of well characterized lateral pnp’s and the large offsets inherent to CMOS opamps. Another problem tackled in this chapter is related to the realization of an 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 amplifier. This example shows that low power is a relative term which has to be adapted to the application.