Analog-to-digital converters are pervasively used in many applications ranging from low frequency applications such as sensor interfaces to high frequency applications such as A/D converters used for wireline and wireless communication systems. One commonly used A/D architecture is an oversampled A/D converter that uses a sigma delta modulator. A sigma delta modulator is generally a feedback loop that includes one or more integrators in a forward path followed by a low-resolution quantizer, the output of which is subtracted from the input to form an error signal. The quantizer output, which can have as low as one bit of resolution, is then decimated using a digital decimator to produce a multi-bit output.
One of the well-known properties of the sigma-delta modulator is that the quantization noise of the modulator is shaped by the loop, which yields a vast improvement in the signal-to-noise ratio with respect to a Nyquist rate A/D converter with the same quantizer resolution. For example, in a PCM A/D converter, each doubling in frequency yields a 3 dB improvement in the SNR. In a sigma-delta converter, however, each doubling in frequency ideally improves the SNR of the A/D converter by approximately, (6 L+3) dB, where L is the order of the sigma-delta modulator. As such, sigma-delta modulators can provide SNRs of well over 100 dB for audio and low frequency applications.
One common way of implementing a sigma-delta A/D converter is by using a switched capacitor circuit. At its most basic level, switched capacitor circuits perform analog signal processing in the charge domain by sampling charge on capacitors. CMOS processes are particularly suited for such circuits. By using a combination of feedback amplifiers, switching devices and ratioed capacitors, various sampled analog transfer functions, including the integrator used for sigma-delta modulators may be accurately represented even in the presence of a high amount of absolute component value variation.
With respect to low frequency and DC applications, switched capacitor based sigma-delta A/D converters pose a few issues with respect to noise and offset. While the oversampled nature of the sigma delta modulator reduces the effect of kT/C thermal noise of the switches by spreading this noise across a higher bandwidth, issues of low frequency flicker noise and offset still remain. Switched capacitor techniques such as correlated double sampling may be used to mitigate these effects; however, as effective resolution of the converter increases, second order effects begin to limit the effective resolution of the A/D converter.