Two-wire millivolt to milliampere signal converter

Claim 2

A two-wire system for converting small DC signals into milliampere signals comprising: means for sensing DC signals from a very high impedance probe source, an operational amplifier including means constituting a high input impedance coupled to said means for sensing and means constituting a low impedance output, means constituting a current sensor having an input coupled across the output of said operational amplifier, a pair of conductive paths for coupling across a power source, means constituting a high current path and preregulator coupling one of said conductive paths to the input of said current sensor, B plus means for furnishing a regulated and temperature compensated positive supply voltage from the first one of said conductive paths to said operational amplifier, B minus means for furnishing a regulated and temperature compensated negative supply voltage from the other of said conductive paths to said operational amplifier, said current sensor having an output signal current applied to the other of said conductive paths and including means for applying fractional feedback to the input of said operational amplifier.

Claim 3

The circuit system of claim 1 wherein said high current path and preregulator means includes a zener diode and a plurality of diode rectifiers.

Claim 4

A two-wire system for converting small DC signals into milliampere signals comprising means for sensing DC signals from a very high impedance probe source, an operational amplifier having a high input impedance coupled to said means for sensing and a low-impedance output, means constituting a current sensor having an input coupled across the output of said operational amplifier, a pair of conductive paths for coupling across a power source, means constituting a high current path and preregulator coupling one of said conductive paths to the input of said current sensor, B plus means for furnishing a regulated and temperature-compensated positive supply voltage from the first one of said conductive paths to said operational amplifier, B minus means for furnishing a regulated and temperature-compensated negative supply voltage from the other of said conductive paths to said operational amplifier, said current sensor having an output signal current applied to the other of said conductive paths and in addition applying fractional feedback to the input of said operational amplifier, said B plus means comprising a transistor including an emitter connected to said operational amplifier, a collector connected to the first one of said conductive paths, and a base, a resistor connected across said first one of said conductive paths and said base defining a current limiter for said transistor, and a zener diode and a plurality of diode rectifiers coupled to the load of said operational amplifier and respectively defining voltage regulation and ambient temperature compensation.

Claim 5

A two-wire system for converting small DC signals into milliampere signals comprising means for sensing DC signals from a very high impedance probe source, an operational amplifier having a high input impedance coupled to said means for sensing and a low-impedance output, means constituting a current sensor having an input coupled across the output of said operational amplifier, a pair of conductive paths for coupling across a power source, means constituting a high current path and preregulator coupling one of said conductive paths to the input of said current sensor, B plus means for furnishing a regulated and temperature-compensated positive supply voltage from the first one of said conductive paths to said operational amplifier, B minus means for furnishing a regulated and temperature-compensated negative supply voltage from the other of said conductive paths to said operational amplifier, said current sensor having an output signal current applied to the other of said conductive paths and in addition applying fractional feedback to the input of said operational amplifier, said B plus means comprising a transistor including an emitter connected to said operational amplifier, a collector connected to the first one of said conductive paths, and a base, a resistor connected across said first one of said conductive paths and said base defining a current limiter for said transistor, and a zener diode and a plurality of diode rectifiers coupled to the load of said operational amplifier and respectively defining voltage regulation and ambient temperature compensation.

Claim 6

The circuit system of claim 3 wherein said B minus means comprises a current regulator diode having one end connected to the other of said conductive paths and passing a fixed amount of current therefrom, a voltage regulator zener diode connected to the other end of said current regulator diode, a second plurality of diode rectifiers connecting the other end of said zener diode with said load, and a second transistor including a base and collector coupled across said current regulator diode, and an emitter connecting the negative supply voltage to said operational amplifier.

Claim 7

The circuit system of claim 4 including means constituting a potentiometer across said negative voltage supply for adjustment of zero balancing.

Claim 8

The circuit system of claim 5 wherein said means for adjustment of zero balancing comprises a voltage-divider resistance network having one end connected to the load and the other end to the emitter of said second transistor, and a third transistor including a base connected to an intermediate portion of said resistor network, a thermistor network coupling the emitter of said third transistor to the emitter of said second transistor, a fourth transistor including a base coupled to the collector of said third transistor, a collector coupled to the emitter of said second transistor, and an emitter connected to the fixed portion of said potentiometer, the movable portion of said potentiometer being connected to one end of said sensing means.

Claim 9

The circuit system of claim 5 including means to compensate for variations in temperature of samples being measured.

Claim 10

The circuit system of claim 7 wherein said means to compensate comprises a thermistor having one end connected to the feedback input to said operational amplifier and the other end to the load.

Claim 11

The circuit system of claim 8 including means to compensate for zero shift of said voltage supply.

Claim 12

The circuit system of claim 9 wherein said means to compensate for zero shift comprises a thermistor connected at one end to the load and the other end being coupled to the collector of said third transistor and the base of a fifth transistor whose collector is connected to the emitter of said second transistor and whose emitter is connected to the load through a resistance.

Claim 13

The circuit system of claim 3 including a rectifier in one of said conductive paths to protect the circuit against reversals in polarity.

Claim 14

The circuit system of claim 11 including a resistor in series with the conductive path containing said rectifier in order to limit power dissipation of said current limiting transistor if low resistance loads are used.