1 Introduction

One way to classify a Current Sense Amplifier (CSA) is based on whether it is able to measure current in both directions. Then it can be put into one of two categories - unidirectional or bidirectional. A unidirectional device only linearly responds to current flowing in one direction, with its output moving in one direction in proportion to the input differential signal. A current flowing in the opposite direction causes the output to collapse to one of the supply rails, normally ground. Figure 1-1 illustrates such a scenario.

For a bidirectional CSA, current flowing in either direction is allowed. The output of the device moves off of a quiescent output level, in proportion to the input differential signal. The fact that bidirectional CSA output is able to move up toward supply or down toward ground implies that the quiescent output level corresponds to zero current. In these devices, there is typically one or two output reference pins. The output is level-shifted by driving the reference pins with a suitable source. Figure 1-2 shows the same bidirectional input is accurately reproduced. A bidirectional CSA can be configured as unidirectional by setting the quiescent output at or close to either supply rail.