| 摘要 |
<p>Analog-to-digital converters are subject to various error inducing factors such as offset errors, scale factor instability, quantisation errors, bit weighing errors, and particularly where samples from two or more multiplexed input signal channels are to be digitized, cross-talk errors in the form of signal residues from one channel which affect subsequent channels. The analog-to-digital conversion apparatus described herein comprises a multiplexer for sampling a series of input channels in a sequence which repeats several times over a predetermined digital value update interval, an analog-to-digital converter which digitizes the samples, and an adder/memory combination for summing the samples of each signal during the update interval. Offset errors are reduced by an auto-zeroing technique in which half the samples taken in each update interval are inverted prior to digitization and re-inverted after digitization. Preferably, to reduce cross-talk, the auto-zeroing does not just comprise inverting and re-inverting for one complete half of the update interval - rather the auto-zeroing 'on' signal comprises a chosen binary sequence with one bit for each sample. The auto-zero 'on' periods still sum to half the update interval but are distributed over that interval, in effect, so as to decorrelate the channels. By making the update interval equal to the period of some dominant noise component which may appear in the input channels, i.e. the gyro wheel noise in the case of an inertial navigation system, this noise component can be averaged out by the auto=zeroing and sample summation.</p> |
