Editor’s note: Part 1 of this two-part series discussed the nuances of current sense resistors. Part 2, here, discusses the design and use of amplifiers to boost the voltage developed across them to usable levels.
Current sense resistors, also called shunts, are the technology of choice for measuring current flow. In order to not adversely affect current flow, they have a small value that produces a proportionally small voltage across them. As a result, designers must utilize circuitry that amplifies this small voltage for upstream conversion by an analog-to-digital converter (ADC).
The small voltage across the shunt resistor must usually be boosted from tens or hundreds of millivolts to tenths of a volt or volts. This task is frequently performed by an operational amplifier (op amp), or a current sense amplifier. A current sense amplifier is a specialized op amp with an added laser trimmed, precision resistor network incorporated into the device to set its gain. Typically, amplifier voltage gains are on the order of 20 to 60, and sometimes even larger.
The current sense amplifier may or may not include the current shunt resistor in the same package. For high-power applications, an external shunt resistor is preferable because of the power dissipation which results in heat.
The most common signal chain configuration for monitoring current flow includes the shunt resistor, an analog front-end (AFE), an analog-to-digital converter, and a system controller (Figure 1). An AFE, such as an operational amplifier or dedicated current sense amplifier, converts the small differential voltage developed across the shunt resistor into a usable voltage for the ADC.
Figure 1: The easiest way to measure current flow is with a current shunt resistor (far left), across which a voltage develops that’s proportional to the current flowing through it. An AFE amplifies the low voltage across the shunt resistor in order to use the ADC’s full measurement range. (Image source: Texas Instruments)
There are two basic ways to wire a shunt resistor into a circuit for low and high side current measurements. Both approaches have advantages and disadvantages.
Low side current measurements
A low side current measurement places the current shunt resistor between the active load and ground. The most appropriate circuit for making low side current measurements is shown in Figure 2. The circuit uses a Texas Instruments INA181 current sense amplifier, although many other amplifiers can also be used for low side measurements.
Figure 2: A low side current measurement circuit using a Texas Instruments INA181 places the current sense resistor between the active load and ground. (Image source: Texas Instruments)
Low side current measurements are simple to implement because the sense voltage across the current shunt resistor is ground referenced. This configuration allows the current sense amplifier to be a low voltage part because the voltage being sensed is only on the order of millivolts above the ground reference. In this configuration, the sense voltage does not ride on a higher voltage so no common mode rejection is required. The low side measurement method is the simplest, lowest cost method to implement.
The disadvantage of low side current measurement is that the load is no longer ground referenced due to the placement of the shunt resistor, causing the low side of the load to sit several millivolts above ground.
No ground reference can become a problem if there’s a short circuit between a load and ground. Such a short can occur, for example, if a metal-enclosed load such as a motor has a winding short to its ground referenced case. The current sense resistor may not be able to detect such a short circuit.
Additionally, the amplifier’s common mode input voltage must include ground to make a low side measurement. This is usually not a problem for amplifiers running on positive and negative power supplies, but it can be an issue for those with a single supply. Therefore, a common mode voltage range that contains ground becomes an important criterion when selecting an appropriate amplifier to make low side measurements.
There’s one more important aspect to making low side current measurements. Note that the Texas Instruments ADS114 ADC in Figure 2 is referenced directly to ground, and that the ADC’s low side input node is located close to the INA181 current sense amplifier’s input ground reference connection.
For current sensing using small voltages developed across low resistance shunt resistors passing high load currents, it’s important to remember that all grounds may not be at the same potential. It’s quite easy to develop millivolts of differential between one ground point and another in a system when the ground networks or ground planes are carrying the high currents associated with many power applications. As a precaution, always be sure to keep related ground references wired together in very close proximity to each other to minimize voltage differences between them.
To remove this source of error, the ADC’s ground reference pin must be connected in close proximity to the low side of the current sense resistor and the low side input of the current sense amplifier. The connection point simply cannot be any convenient part of the ground plane. To be doubly sure, make a note of this requirement directly on the schematic, and show a star connection for the ground references to really underscore the point.
Likewise, the current sense amplifier’s input offset voltage disproportionately affects the amplification accuracy the when the voltage across the current sense resistor is small. For that reason, it’s best to select an amplifier with a very low input offset voltage. The INA181 amplifier shown in Figure 2 above has an input offset voltage of ±150 microvolts for low side measurement configurations where there’s no common mode voltage present.
Despite the few disadvantages, the low side current measurement configuration is a good choice if the load need not be ground referenced, and if internal short circuits between the load and ground are either not an issue or need not be sensed by the current measuring circuitry.
However, for designs that must meet functional safety requirements, the high side current measurement technique is a better choice.
High side current measurements
A high side current measurement inserts the current shunt resistor between the power source and the active load as shown in Figure 3, using a Texas Instruments INA240 current sense amplifier as an AFE. This device’s common mode input voltage can greatly exceed its supply voltage, making it a good choice for high side current measurements.
Figure 3: A high side current measurement circuit places the current sense resistor between the power source and the active load. (Image source: Texas Instruments)
High side current measurements have two key advantages over low side measurements. First, it’s easy to detect a short circuit originating from within the load to ground because the resulting short circuit current will flow through the current shunt resistor, developing a voltage across it. Second, this measurement technique is not ground referenced so differential ground voltages developed by high currents flowing through the ground plane will not affect the measurement. However, it’s still a good idea to carefully place the ADC’s ground reference connection close to the amplifier’s ground.
The high side current measurement technique has one main disadvantage. As discussed above, it requires that the current sense amplifier have high common mode rejection because the small voltage developed across the current shunt rides just below the load supply voltage. Depending on the system design, this common mode voltage can be quite large. The INA240 current sense amplifier in Figure 3 has a wide common mode range of -4 to 80 volts.
Integrated gain resistors or not?
Figures 2 and 3 illustrate low and high side current measurement configurations, both employing current sense amplifiers with integrated gain setting resistors. These integrated resistors offer many design advantages including simplification of the design, reduction of board components, and laser trimmed gain accuracy. The one big drawback to the use of such amplifiers is that the gain is permanently set at the factory. This isn’t a problem if the gain setting is appropriate for a given application. However, if the application calls for a unique gain because the shunt resistor’s value was selected to satisfy other criteria, then an op amp combined with discrete resistors is a better choice.
Figure 4: High side current measurement configuration using discrete resistors and an op amp. (Image source: Microchip Technology)
In this circuit, the amplifier gain is set by the ratio of R2 divided by R1. Also note that R1* = R1, R2* = R2, and that the current shunt resistor RSEN should be much, much smaller than either R1 or R2. That’s not usually a problem because the current shunt resistor’s value is generally on the order of milliohms or even fractions of a milliohm for very high current applications.
The equations in Figure 4 make it clear that the use of an op amp and discrete resistors requires a bit more component specification than when using current sense amplifiers with internal gain setting resistors.
Current sense amplifiers transform the low voltages developed across shunt resistors into larger voltages more compatible with ADC conversion. There are two types of current sensing measurement possible: low side and high side. Low side measurements insert the current sense resistor between the load and ground, while high side measurements insert the current sense resistor between the power supply and the load. Both low side and high side measurement configurations have advantages and disadvantages, so the choice requires some thought and consideration for a given application.
When measuring current, it’s possible to use either a purpose-built current sense amplifier with the gain set at the factory using integrated, laser trimmed resistors, or an appropriate op amp and discrete resistors. The first choice reduces the number of board components and simplifies the AFE’s design. However, if the AFE design requires a custom gain to accommodate a specific value of shunt resistor and ADC input voltage range, the second choice is more appropriate.