1 Introduction

Current sense amplifiers are typically powered with a single supply and designed to function with an input common-mode range that far exceeds supply voltage. At the same time, differential input voltage, V_sense, needs to be precisely amplified without distortion. Ideally, the range of V_sense is only limited by supply voltage, and extends all the way down to and including ground.

A unidirectional CSA only linearly responds to current flowing in one direction, with the 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 the best output response of a unidirectional CSA with bipolar differential voltage input. The CSA precisely amplifies the input signal without any distortion or delay when V_sense is positive. When V_sense is negative, the output collapses to ground or 0 V.

Figure 1-2 depicts the DC input-output transfer function of the unidirectional CSA. Assuming a power supply voltage V_s, the best transfer function is shown by the solid black piecewise linear curve. When V_sense is within the range of ground and V_s/Gain, where Gain stands for the CSA gain, the input-output transfer curve is a straight line that pass through (0, 0) and (V_s/Gain, V_s). When V_sense is below 0 V, the output is stuck at ground; when V_sense is greater than V_s/Gain, the output is stuck at V_s.

Unless special circuits are involved, such as charge pumps, there is normally a limit to the smallest V_sense possible which does not include 0 V, due to practical implementations. Below the limit of the smallest V_sense, the amplifier output can no longer be regarded as a reliable representation of input.

Typical CSA output is not capable of swing rail-to-rail. A more realistic input-output transfer function is depicted by the red curve, which is exaggerated to show the deviation from the ideal curve.

When V_sense approaches zero, the output encounters swing to ground limit. The continued decrease in V_sense can no longer cause the output voltage to decrease in proportion, which eventually stops at a level that is higher than ground. The output swing to ground characteristic imposes a limit on how low V_sense can be. Similarly at the high end, swing to power supply imposes an upper limit on V_sense.

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 are typically one or two output reference pins. The output is level-shifted by driving the reference pins with a suitable source. Figure 1-3 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.

It is customary to configure a bidirectional CSA quiescent output level to be midway between ground and V_s. A V_ref at mid-supply allows symmetric bidirectional input with respect to ground. Taking the CSA of Figure 1-2 and configuring it as bidirectional while keeping all other characteristics unchanged, the transfer function as shown in Figure 1-4 is attained. The linear input range now includes 0 V.