Switching systems which comprise a differential switched capacitor circuit, having outputs coupled to the inputs of an operational amplifier, are well known.
The operational amplifier of such systems usually has some level of noise. This may be caused by thermal or flicker noise of the operational amplifier's internal components or shot noise in its input currents. This noise of the operational amplifier significantly reduces the dynamic range of the system and thus careful operational amplifier design is required.
In mixed-signal integrated circuits (ICs), that is ICs which process both analog and digital signals, digital spikes may migrate to the analog part of the IC, with the result that the noise level of the operational amplifier increases. In practice, it is virtually impossible to avoid digital spike penetration to the analog part and thus, noise in such mixed-signal ICs is particularly problematical.
In oversampling systems, only the low frequency noise components, which fall into the system's bandwidth, are problematical. However, switched capacitor circuits sample the high frequency noises on the capacitors and alias them to low frequencies. Thus, such sampled noises also add to the noise problem.
It would therefore be desirable to design a switched capacitor circuit which reduces the aliasing of high frequency noise and spikes to low frequencies and which also reduces low frequency noise.
A number of solutions, which attempt to address the noise problems, have been developed. One solution is described in a paper by Mike Rebeschini et al, entitled `A 16-bit 160 kHz A/D Converter Using SigmaDelta Modulation` (IEEE Journal of Solid State Circuits, Vol. 25, #2, April 1990). This solution is commonly called the `Auto-Zero Circuit` or `Noise Correlation Circuit`.
The auto-zero circuit compensates for the error signal on the inputs of the operational amplifier. The error signal may be due to offset and/or low frequency noise. The compensation is based on measuring the operational amplifier input error voltage in one phase and using it for input error compensation during the other phase. It is assumed that the input error signal does not significantly change between phases.
In mixed-signal systems, digital spikes and high frequency noise change rapidly between the measurement phase and the compensation phase. Hence, in such systems, the noise cannot be properly compensated and this approach cannot prevent aliasing noise to low frequencies.
An additional disadvantage of the auto-zero circuit is that it requires the operational amplifier to swing from the signal level to analog ground, each phase. This produces high bandwidth and slew rate requirements. Implementations are known which meet the slew requirements, however, these known implementations are extremely sensitive to stray capacitance.
A number of different noise reduction circuits are also known. However, they all work on the same principle as that described above: the noise is assumed to be DC offset or low frequency noise and the noise during a present phase is compensated according to the noise during the previous phase. As described above, such circuits suffer from severe disadvantages, since they do not reduce high frequency noise aliasing and cannot compensate for noise due to digital spikes in mixed-signal systems