Correlated double sampling

Two different schemes are used for modulation chopper stabilization, which works similarly to an AM radio, and correlated double sampling (CDS), which performs two measurements of the noise only and the noise plus signal [19]. Fig. 6.1.11 shows conceptual diagrams of the two techniques. Although the additional circuitry looks complex, actual implementations are surprisingly simple [20, 21]. 

Fig. 6.1.11 Modulation techniques a) chopper stabilization, b) correlated double sampling...

Various circuits are used to simulate a large resistor with active components that take up significantly less chip area. A particularly simple solution consists of a switch that periodically charges the sense capacitors to a known value. This solution can be conveniently implemented in combination with correlated double sampling. Since the sensor capacitance is often small, the noise aliased by this resetting can be significant [22]. 

A monolithic three-axis accelerometer with three independent capacitive readout circuits on a single chip is described elsewhere [7] (Fig. 6.1.14). The circuit is similar to Fig. 6.1.10 and achieves 0.085 aF/v/Hz resolution with 100 fF sense capacitors. The noise is actually dominated by Brownian noise in the sensor itself, as tests in vacuum demonstrate. The actual capacitance resolution is therefore somewhat better than stated. This circuit uses correlated double sampling (CDS) for biasing and to reject flicker noise. 

Fig. 6.1.15 Timing of correlated double sampling (CDS) a) circuit reset, b) sampling noise cancellation, c) sensing...

Many other integrated solutions are documented in the literature. These include single-ended and differential solutions employing chopper stabilization [1, 20, 24, 31], circuits using correlated double sampling [2, 3, 32] and applications for capacitive readout circuits and corresponding implementations [15]. 

Fig. 6.7. Circuit for correlated double sampling. The preamplifier input voltage is reset to V , and the output voltage to zero by closing and then opening the reset switch. The charge is then transferred and the output voltage is sampled. With this approach the output measures only the change in voltage caused by the charge transfer operation. kTC voltage noise caused by the reset operation is not transmitted to the output...

If correlated double sampling is not used, besides background noise the most important source of noise for the series-parallel scan CID is likely to be kTC noise. If this is the case then the maximum signal frequency for which we can design the device to obtain BLIP operation will be... 

Figure 7.8 Sampling methods to reduce or eliminate noise in the output, (a) Correlated double sampling (CDS), (b) Fowler sampling, (c) Sampling up the ramp.

Figure 7.9 Integrating beyond saturation causes a reduction in apparent signal amplitude for system incorporating correlated double sampling.

Exceptions and Precautions In our derivation we assumed that the noise was determined by single-sampling - the simplest method. Determining noise by correlated double sampling (described in Section 7.2.5.1 and Figure 7.8a) will... 

Compatible Sampling Techniques Standard Correlated Double Sampling (CDS) Delta Reset CDS Multiple CDS... 

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