How to make precision measurements on a nanopower budget
Heightened accuracy and speed in an operational amplifier (op amp) has a direct relationship with the magnitude of its power consumption. Decreasing the current consumption decreases the gain bandwidth; conversely, decreasing the offset voltage increases the current consumption.
Many such interactions between op amp electrical characteristics influence one another. With the increasing need for low power consumption in applications like wireless sensing nodes, the Internet of Things (IoT) and building automation, understanding these trade-offs has become vital to ensure optimal end-equipment performance with the lowest possible power consumption. In the first installment of this two-part blog post series, I’ll describe some of the power-to-performance trade-offs of DC gain in precision nanopower op amps.
You probably remember from school the classic inverting (Figure 1) and noninverting (Figure 2) gain configurations of op amps.
These configurations resulted in inverting and noninverting op amp closed-loop gain equations, Equations 1 and 2, respectively:
where ACL is the closed-loop gain, RF is the value of the feedback resistor and R2 is the value of the resistor from the negative input terminal to signal (inverting) or ground (noninverting).
These equations are a reminder that DC gain is based on resistor ratio, not resistor value. Additionally, the “power” law and Ohm’s law show the relationships between resistor value and power dissipation (Equation 3):
where P is the power consumed by the resistor, V is the voltage drop across the resistor and I is the current through the resistor.
For nanopower gain and voltage divider configurations, Equation 3 tells you that, in order to minimize power dissipation, you need to minimize the current consumption by the resistor. Equation 4 helps you understand the mechanism to do that: