Abstract:
A constant current source employs a correction signal which is multiplied with the output of the current source to correct for amplitude variations and thereby maintain a constant current output level.

Description:
BACKGROUND OF THE INVENTION 
     Current sources are generally of the voltage-to-current type converters. In such converters the output current is directly proportional to the input voltage. Unless the input voltage is from a highly stable voltage source, the output current will be subject to the variations and fluctuations of the input voltage. 
     It is important in many operations, such as in resistivity measuring apparatus and systems, to have a source of current that exhibits a constant-current level free of any such variations and fluctuations. Accordingly, it is a specific feature of the present invention to provide for such a constant-current source. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward a current source for use whenever the current output needs to be maintained at a constant level. A current source is applied to a resistive element and a reference voltage is produced representative of the voltage differential across the resistive element. A fixed DC voltage is provided and the difference between this fixed DC voltage and the reference voltage is integrated to produce a correction signal. This correction signal, when multiplied with the output of the current source, corrects for amplitude variations in the current source. In this manner, the current through the resistive element is maintained at a constant level regardless of instability of the current source. 
     In a further aspect of the invention, the reference voltage is produced by measuring the voltage differential across the resistive element and rectifying such voltage differential measurement to remove any DC ripple present. 
     In a yet further aspect, the resistive element includes a bank of resistors and means for establishing the value of the constant-current flow by the selection of at least one of the resistors in the bank of resistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is an electrical schematic of the constant-current source of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the FIGURE, a variable-frequency RMS current source 10 is connected through inverter 11 to one input of a multiplier 12. The other input to multiplier 12 is a correction signal supplied by the integrator 25. This correction signal, when multiplied with the output from the current source 10 corrects for amplitude instability of the oscillator of current source 10 and results in a constant-current output on line 15. This correction signal is derived in the following manner. The current output from multiplier 12 is applied through an amplifier 13 to a bank of resistors 14. These particular resistors are selected to vary the current line 15 in incremental steps from one micoampere to 10 milliamperes for example. The desired value of current then passes through the amplifier 17 to a differential amplifier 21 which operates to measure the voltage differential across the bank of resistors 14. This voltage differential is a D.C. voltage with a small amount of ripple which is removed through rectification in digital converter 24. This differential voltage is then applied as a reference voltage to the integrator 25. Also applied to integrator 25 through an inverter 27 is a fixed D.C. voltage from the supply 26. Integrator 25 operates to integrate the difference between the rectified reference voltage and the fixed D.C. voltage. The output of integrator 25 is utilized as the correction signal to multiplier 12 for correcting amplitude variations in the current output from oscillator 10. In this manner a constant-current output is maintained regardless of oscillatory instability of the current source itself. 
     Having described the constant-current source of the present invention in conjunction with the circuitry illustrated in the FIGURE, it is to be understood that such circuitry is merely representative of one embodiment. In accordance with such embodiment, the following sets forth specific types of circuit components. 
     
         ______________________________________Reference Designation               Description______________________________________Oscillator 10       DG 502 TektronixInverters 11 &amp; 27   TL084Multiplier 12       AD534Amplifier 13        LH0002Resistor 14a        1MResistor 14b        100KResistor 14c        10KResistor 14d        1KResistor 14e        100Amplifiers 17 &amp; 21  OP15A/D Converter 24    AD442KIntegrator 25       TL08410 VDC Supply       MC1404A______________________________________ 
    
     It is to be understood that the foregoing description relates to only a preferred embodiment of the invention and that modifications or alterations may be made without departing from the spirit and scope of the invention as set forth in the appended claims.