2 Example 1: Output Swing Limitation from V_CM in an Op-Amp for Low Side Current Sense

As an example, let’s assume that we are working with OPA391 operational amplifier, which we have in a gain of 100 V/V, powered with a 5.0-V single supply. Let's say we want to measure current between 0 and 50 A, and we choose a 1-mΩ shunt resistor. This means we will see differential input in the range of 0 to 50 A × 1 mΩ = 50 mV. The minimum V_CM value for OPA391 is 0.1V below ground, so our input conditions are in agreement with the data sheet requirements. However, OPA391 A_OL output conditions are specified in the range of –V_s + 0.1 V < V_out < +V_s – 0.1 V. Therefore, between 0 and 0.1 V and 4.9 V to 5 V of output the op-amp may encounter some non-linearity, which is undesirable. We can solve the issue quite easily:

• We can level-shift the input common mode up by 1 mV (which is amplified by the closed loop gain), or level shift -V_s down by 100 mV. Level shifting the input can be achieved through a simple voltage divider circuit, or providing headroom for the output can be accomplished by providing a small negative rail. If no negative rail is present in the system, a negative charge pump like the LM7705 solves that problem in a single IC. In either case, it will put V_out at least 100 mV above –V_s under 0 current condition, which resolves the output swing issue on the low end. Take note that either solution also exceeds the maximum linear operating range on the high end, which is in violation of the output voltage swing specifications. We can solve that issue by decreasing the gain of the circuit or the value of the shunt resistor slightly to bring V_{out (max)} at or below +V_s – 100 mV.

While the complementary input stage described offers an excellent solution to the input common mode problem, it is important to keep in mind that the transition between each pair will generate a change in the input offset voltage of the amplifier, also known as input crossover distortion. The transition can be eliminated by keeping both pairs conducting at all times, but this is often avoided because of the excessive power dissipation required. The input crossover distortion can be circumvented more elegantly by using a zero-crossover amplifier (Zero-crossover Amplifiers: Features and Benefits tech note). These amplifiers use a single transistor pair and an integrated charge pump to push the internal voltage supply enough beyond the nominal value to remain in linear operation.