Bipolar digital-to-analog converter

Claim 1

A bipolar digital-to-analog converter comprising A. a reference source having two terminals, one being designated common ground, B. a unipolar conversion means having 1. a digital-to-analog network connected across said terminals of said reference source, 2. an input to said network for digital information that is to be converted to an analog voltage, and 3. an output terminal receiving from said network a voltage, with respect to common ground, which is the analog of digital information at said input to said network, C. a difference amplifier 1. having first and second input terminals and an output terminal, 2. having a feedback resistive element connected from said amplifier output terminal back to said second amplifier input terminal, 3. providing, at said output terminal thereof, a voltage representative of the difference between the signals at said input terminals thereof, said voltage representing zero when the signals at said input terminals are balanced and shifting either positively or negatively depending on which of said input terminals is positive with respect to the other, D. means for providing two isolated output terminals of said conversion means and carrying signals of the same polarity, said means comprising 1. a resistive network having a. a first resistive element connected between said network output terminal and said first amplifier input terminal, b. a second resistive element connected between said network terminal and said second amplifier input terminal, and c. a third resistive element connected between said second amplifier input terminal and said common ground, and E. switching means connected between said first amplifier input terminal and said common ground and having an input connection for selectively actuating said switching means in response to digital information, 1. said switching means having at least two states and operative in one state to cause said first amplifier input terminal to follow a signal from said conversion means more closely than said second amplifier input terminal, and in the second state to cause said second amplifier input terminal to follow the signal from said conversion means more closely than said first amplifier input terminal, so that with said switch in one of said two states said difference amplifier provides at said output terminal thereon a voltage of the same polarity as said voltage from said conversion means and in the other of said two states said difference amplifier provides a voltage of the opposite polarity from said voltage from said conversion means.