SBOA535 February 2022 INA190
As applications continue to become more advanced in scope, designers find themselves needing to monitor greater ranges of current in their designs. From the scaling of high-power applications requiring the need for high current flow, to the advancement of semiconductor content allowing resolutions to be successfully measured down into the nanoamp range, designers are constantly looking for methods and topologies that allow them to achieve wider ranges of measurement.
There are several ways to define the range of an application, and these may be referred to either the input or the output of a certain device. In general, the ratio of largest-to-smallest input measurement range able to be measured by a device in an application is known as the dynamic range of the application, while the largest measurable quantity is called the full-scale range. Full-scale range is often used to describe the maximum value attainable in an analog-to-digital converter (ADC) design. For example, a current sense amplifier looking to achieve 5-A to 25-A measurement range would have a dynamic range of 5:1, and would have a full scale input range of 25 A. Full scale; however, could also refer to the highest possible output achievable by the amplifier, commonly referred to as the full-scale output range.
For current sense amplifier designs, the objective in the simplest sense, is to transduce a current signal range through a shunt resistor into a voltage range, and provide a chosen gain that will map that signal to the maximum output voltage range capable with the amplifier. For most current sense amplifiers, the output range able to be utilized spans from a few millivolts above ground to a few millivolts below the supply rail (although for optimal results, linear operating range of the amplifier should also be considered). This straightforward design plan begins to break down; however, as the DR of the application grows wider. From here, each design aspect can be discussed to understand the tradeoffs made and how the DR is affected.
When designing with a current sense amplifier, several degrees of freedom exist to form the design, with the typical equation relating the current measured in the shunt to the output of the amplifier given as Equation 6:
Observe from Equation 6 that there are essentially three options presented to the designer to help form their design: the magnitude of the supply voltage, the gain option chosen, and the sizing of the shunt resistor.
In designs where wide DR is necessary, maximization of the supply voltage is recommended to provide the widest output range possible for the design, although downstream circuitry may also influence this choice. Supplying a VS lower than the recommended maximum directly reduces the dynamic range possible for a single device.
Choosing an amplifier with larger gain is typically a tradeoff: this allows more signal integrity to be generated for a smaller signal against the offset voltage (larger gain choices also lead to increased resolution when digitizing the signal), thus reducing error in the lower region. However, this larger gain also results in more rapid maximization of the output of the amplifier, or in short, the size of the allowable dynamic range becomes smaller. Therefore, gain increase is typically more useful in precision range designs, where the dynamic range of measurement is reasonably small, or with amplifiers whose supply voltage range may also be extended to support the output on the higher end.
As an example, consider a system utilizing INA293 with a 5-V supply, where the desired range of the design is from 20 mA to 1 A. Disregarding common-mode rejection ratio (CMRR), if the designer were to implement a 200-mΩ shunt, the generated shunt voltage at the 20-mA condition is 4 mV, and, according to the Current Sense Amplifier Comparison and Error Tool, the error for the A1 variant at this point is 3.86%. Assuming better accuracy is needed, consider a step up to the A2 variant, which, for the same measurement case, now reduces the error to 2.12%. The tradeoff to this device migration, however, is that by stepping up to the higher gain, the design has diminished the maximum allowable range of which the amplifier can measure. In this case, while the error at the lower bound was reduced, this also invalidates the design, as 1 A is no longer an achievable measurement, with the A2 device reaching saturation at 485 mA for a 200-mΩ shunt, as demonstrated in Equation 2 and Equation 3.
Therefore, for wider ranges of measurement that are the subject of this paper, it is observed that smaller gains allow for maximization of dynamic range, so A1 variants are typically chosen for these types of designs. This example is summarized in Table 1-1.
| Part Number | Gain (V/V) | Error at 20 mV (%) | Maximum Meas. Load (A) |
|---|---|---|---|
| INA293A1 | 20 | 3.86 | 1.2125 |
| INA293A2 | 50 | 2.12 | .485 |
The shunt resistor is the final aspect that needs to be considered in the given system. The challenge of shunt resistor design is optimizing error at the low end versus shunt power loss at the high end. For a specific current point, Ohm's law is clear, to generate additional voltage signal against the offset voltage, the resistance of the shunt resistor must increase. This, however, comes at the expense of resistance losses in the form of heat, which in many cases may be become a challenge to manage successfully for a given design.
Take for example, an arbitrary, high-current application design using the INA240 to measure a maximum current of 100 amps. Taking the swing limitations of the INA240 into account, and assuming that the device is powered by a supply of 5 V, the maximum worst-case output the INA240 is capable of delivering is 4.8 V, and assuming the A1 variant (GAIN = 20 V/V) is used, the maximum possible shunt able to be designed in is:
This result shows that up to a 2.4-mΩ shunt may be chosen without risking device saturation (although some margin from the rail is typically advised), but this does not necessarily mean that this is the optimal shunt for the design. While on paper, this shunt choice maximizes signal integrity of the load under measurement, the power dissipated in the shunt at the maximum current level is 24 W.
Finally, as per the last example, the needed dynamic range may also be examined, as if a larger gain may be chosen while still capturing the necessary measurement range, the shunt may be reduced by this same factor.
For many applications, this has the potential to exceed the limits of manageable heat in the system, and the choice of the shunt needs to be revisited, rather than designing in this value. The alternative here is to select a shunt smaller in magnitude than the calculated maximum, as choosing a smaller shunt proportionally reduces the power consumed at the expense of signal integrity, that is, not utilizing the full scale output range achievable by the device. This also reduces the signal integrity of measurements at the lower end, leading to increased error where the offset of the amplifier has the potential to encroach on the accuracy of the measurement.