A wide variety of isolation amplifiers and other isolation circuits are widely used in which input circuitry on one side of an isolation barrier such as an isolation transformer, optical coupler, or pair of isolation capacitors have essentially no DC coupling to terminals of output circuitry coupled to the other side of the isolation barrier. Such circuits find widespread use in medical monitoring equipment and various other applications, including remote temperature sensing industrial process control circuits, in which it is essential that transient voltage disturbances (especially common mode transient voltage disturbances) on one side of the isolation barrier do not produce corresponding transient voltage disturbances in the output circuitry. Such transient voltage disturbances typically are produced by DC-to-DC converter circuits or power supply circuits utilized to independently provide decoupled power supply voltages to the input circuitry and output circuitry. Transient voltage disturbances also can be produced by line voltage transients coupled through isolation transformer winding capacitances from the input to the output side of the isolation barrier. In some equipment, such as medical monitoring equipment, transient voltage disturbances produced from sources such as electro-surgery units may have slew rates of up to 10,000 volts per microsecond. Such instruments may produce radio frequency energy of up to 1000 volts RMS at a frequency of one megahertz. In some of the present assignee's recently introduced isolation amplifiers which utilize a pair of 1 picofarad capacitors as an isolation barrier, peak common mode transient currents of 10 milliamperes may flow through the 1 picofarad isolation capacitors and into the inputs of the sense amplifier stage of the output circuitry. Not only should the circuitry reliably conduct these currents, but it should also continue to communicate accurate signal information (for example, information representing an electrocardiogram waveform).
Some prior sense amplifier circuits are capable of operating in the presence of fairly large common mode transient input voltages that increase by as much as 1000 volts per microsecond.
There is a need for an improved current sense amplifier capable of achieving highly linear performance despite the presence of large, rapidly changing common mode input currents.