accumulated during the summing interval represents N samples of the mean and N divided by the count defining the summing interval is thus the mean of means and so subject to little variance. Digital to analog conversion In general, a DAC converts a digital number, N, representing some analog value into that analog voltage value. To make the conversion the digital number is first loaded into a counter. Then the counter is counted down to zero with a string of pulses equal in number to N. Each pulse of the string is given a known integral, δ. Then the string is integrated to produce N.δ, the sum of the pulses. This is the required analog voltage. In some applications where an analog signal is represented by a series of digital numbers that require conversion to a frequency-modulated stream it may be sufficient to take the stream of pulses (two or three level) resulting from the DAC conversion of each number N in turn and apply that stream through a low pass filter directly to the output. The output before filtering will be a crudely frequency-modulated stream with bursts of pulses proportional in length and number to the analog of N separated by blank intervals between bursts. In order to remove the blank intervals and improve the noise performance the full conversion to analog voltage of each successive N by the DAC described above may be held in a sample and hold circuit then passed to a delta-sigma converter to produce a stream of contiguous bursts each of which has its frequency proportional to its generating N. Decimation structures The conceptually simplest decimation structure is a counter that is reset to zero at the beginning of each integration period, then read out at the end of the integration period. The multi-stage noise shaping (MASH) structure has a noise shaping property, and is commonly used in digital audio and fractional-N frequency synthesizers. It comprises two or more cascaded overflowing accumulators, each of which is equivalent to a first-order sigma-delta modulator. The carry outputs are combined through summations and delays to produce a binary output, the width of which depends on the number of stages (order) of the MASH. Besides its noise shaping function, it has two more attractive properties: simple to implement in hardware; only common digital blocks such as accumulators, adders, and D flip-flops are required unconditionally stable (there are no