To convert a sample of an amplitude-modulated voltage wave into binary pulses, the sample is simultaneously compared in two coders with two sets of threshold voltages of positive and negative polarity, respectively. Either one of the two coders yields in unbroken succession of pulses, having the same polarity as the sample, whereas the other coder produces a combination of both positive and negative pulses. Following inversion of the pulses issuing from one coder, and upon suppression of the pulses of one polarity (e.g. negative), the two pulse trains are combined into a series of bit groups each representing the absolute amplitude value of a respective sample. The final step in the processing of any sample is the generation of a sign bit in accordance with the polarity of the unipolar pulse train emitted by one of the coders; this sign bit is then transposed to the beginning of the corresponding group of amplitude bits.
An integrated circuit incorporating an analog-to-digital (A/D) conversion subsystem having a current mode in which only negative analog currents are converted and a voltage mode in which only positive analog voltages are converted is adapted to produce bipolar digital current and voltage signals. An on-board microprocessor operates the A/D conversion subsystem to perform a conversion in the current mode. If the converted digital value is non-zero, this value is used as the negative digital value of analog input signal. If the result of the conversion in the current mode is zero, a second conversion is made in the voltage mode and used as digital value with a positive polarity. An input circuit having an output impedance substantially equal to the "full scale equivalent" input impedance of the IC defined as the full scale voltage of the IC divided by the full scale current, eliminates the need for special scale factors to correlate the positive and negative conversion values.
In a method of converting an analog input signal to a digital output signal, an analog input voltage is compared with two reference magnitudes which are invariable during the comparing. Both reference magnitudes are derived from an analog signal provided in a previous comparison by adding and subtracting a fixed magnitude representing a quantization step. Both reference magnitudes are simultaneously varied by increments or decrements equal to the steps when the analog input voltage is equal to one or the other of the reference magnitudes. Upon each variation, a pulse is emitted having a plus sign or a minus sign according to whether the analog input voltage reaches one or the other of the reference magnitudes. The number of pulses emitted is algebraically totalled thereby providing a digital indication of the mangitude of the analog input voltage.
