A Current Sensing Tutorial—Part II: Devices

After reading this series of articles on current sensing, you will have a solid grasp of the fundamentals of current sensing, devices that are used for current sensing, how to calculate the accuracy of a solution, and guidelines for printed circuit board (PCB) layout and troubleshooting. This article introduces the four differential amplifiers that typically are used in direct current sensing solutions: operational amplifiers (op amps), difference amplifiers (DA), instrumentation amplifiers (IA), and current shunt monitors (CSM).

Operational amplifiers

The use of an op amp for current sensing is limited by input common-mode voltage. Due to the design of the input stage, input common-mode voltage of such a device is limited by the supply voltage (Voa). Additionally, the large open-loop gain of a traditional op amp requires the device to have feedback, which limits its use to single-ended input signals. Such a configuration necessitates its use to only low-side current sensing. Figure 1 depicts the use of an op amp in a low-side current sensing configuration.

Since the input common-mode voltage of the solution shown in Figure 1 is near ground, the op amp input common-mode range should include ground. It may also be desirable to select an op amp whose output is considered rail-to-rail. This yields the greatest range of load currents that can be accurately sensed. One drawback is any parasitic resistance between the shunt resistor and the ground trace adds to the shunt resistor value (see Figure 2).

The voltage developed by a parasitic resistance in this location (for example, PCB trace, solder joint) will ‘pedestal’ the shunt voltage, thereby introducing error. This parasitic resistance may vary greatly in production. For greater accuracy and consistency, a differential measurement across the shunt resistor is required.

Difference amplifiers

A traditional DA is simply an op amp with a precision trimmed resistor network as shown in Figure 3. The resistors are typically trimmed during manufacturing so that R2/R1 = R4/R3. The differential gain (Adm) of the device is therefore R2/R1. The reference voltage (Vref) is added to the output voltage (Vo).

With respect to current sensing, a DA can have input common-mode voltages outside of the supply voltages due to the resistive divider at the inputs, as shown in Figure 4. This allows one to use a DA for high-side current sensing. However, a DA places a load on the system bus voltage due to its finite common-mode and differential-mode input impedances. This load draws current from the system bus voltage, which introduces uncertainty in the measurement. In order to reduce the measurement error due to these input impedances, they should be significantly larger than the system load impedance.

Since the common-mode voltage of a low-side current sensing solution is near 0V, a DA can be used as shown in Figure 5. This minimizes the effect of the common-mode input impedance, but the differential-mode input impedance is still a factor. The use of a DA for low-side measurements negates the issue caused by parasitic resistance to ground in series with the shunt resistance that was discussed in the op amp section. Finally, DAs have fixed differential gains because the resistor network must be trimmed to maintain good common-mode rejection ratio. Some DAs have an on-chip non-inverting amplifier whose gain can be adjusted. If other gains are required it is advisable to either select such a device or add gain to the output of the DA with an external op amp circuit.

In summary, a DA can be utilized for either high-side or low-side sensing. When used for high-side sensing, error can be introduced by the finite common-mode and differential-mode input impedances. When used for low-side sensing, a DA can negate the issue caused by any parasitic resistance to ground that is in series with the shunt resistance.

Instrumentation Amplifiers

Instrumentation amplifiers are typically composed of a DA output stage with buffered inputs as shown in Figure 6.

The first advantage over a DA is the ability to easily change the differential gain (Adm) of the device using an external resistor (Rg). Secondly, the inputs are connected to the non-inverting inputs of a buffer amplifier. The non-inverting inputs are high-impedance, which translates to almost no load on the system bus voltage allowing for the sensing of small system currents. One disadvantage, however, is that the input common-mode voltage of IAs is limited by their supply voltage. Therefore, IAs typically is utilized for low-side measurements (Figure 7).

It is possible to utilize an IA in a high-side measurement (see Figure 8). The designer must ensure that the system bus voltage is within the input common-mode range of the IA as dictated by its supply voltage.

Current shunt monitors

Current shunt monitors are devices that place little load on a system and allow for sensing current under high common-mode voltage conditions. This ability is gained through the design of unique input stages. A typical op amp common-emitter input stage is depicted in Figure 9, while a common-base input stage of a CSM is depicted in Figure 10.

To illustrate this benefit, let’s look at an example where we compare a DA and a CSM for a high-side measurement (Vcm=70V).