Current Sensing-Part II
One possible CSM solution includes the INA282, whose input common-mode range extends from –16V to 80V. At Vcm=70V, the input bias current is ~25 µA. One possible DA solution includes the INA146. The INA146 can accept input common-mode voltages of ±100V with ±15V supplies. The common-mode input impedance of the device is 55kΩ. Therefore the device will draw 70V/55 kΩ = 1.27mA from the bus supply. This comparison illustrates that a CSM can yield higher accuracy (or greater load current range) at large common-mode voltages.
As illustrated by the same example, DAs can have greater common-mode voltage ranges than CSMs.
Finally, in order to maintain good common-mode rejection ratio and desirable pricing, CSMs typically have fixed gains. The exceptions include current output devices, which require an external precision resistor to set the gain, and digital devices.
Solutions for bidirectional load currents include most DAs, IAs, and CSMs. The datasheet typically indicates whether or not a device can be used in a bidirectional system. One key indicator of bi-directionality is the availability of a reference input, as shown in the DA and IA schematics in Figures 3 and 6, respectively. Placing a voltage on the reference pin of a device references the output to that voltage, as discussed in Part 1 in this series.
Input and output voltage range
Care must be taken to ensure than the input and output voltage ranges of all devices mentioned in this article are within the datasheet specifications. The ranges are listed in the “recommended operation conditions” table of their datasheets. Violating the input range of a device, at least, ensures nonlinear operation. Exposing a device’s inputs to voltages outside of datasheet recommendations can also damage the device depending on duration and severity. The output range of a device is directly dependent on the supply voltages. One must ensure that the product of the input voltage and the gain of the device/circuit are within the output range specification of the device.
This is especially important when selecting the supply voltage for IA solutions. Traditional three op amp IAs, as shown in Figure 6, typically have a plot in the datasheet that shows the output voltage range versus input common-mode range for different supply conditions. An example is shown in Figure 11.
If the INA826 is used for a low-side measurement (Vcm ≈ 0V), the output voltage range of the device extends only from ~0.1V to ~1.2V when Vs = 5V (single-supply), Vref = 0V, and G = 100. This range may be too restrictive to utilize the full input range of a typical ADC.
Powering the device with dual supplies, however, allows for a larger output voltage range, as shown in Figure 12.
Vs = ±5V allows the output to swing from –4.9V to +4.85V when Vcm=0V for gains of 1 or 100. However, depending on the system and application, this may require the addition of a –5V power supply.
Nontraditional IAs such as the INA326 utilizes a unique topology that enables true rail-to-rail input and output voltage ranges. With a single-supply voltage of 5V, the INA326’s input voltage range is –20 mV to 5.1V. The output voltage range can swing to within 75 mV of either supply rail. This makes such devices attractive for low-side current measurements.
In summary, we have presented the four devices that typically are utilized for direct current sensing: op amps, DAs, IAs, and CSMs. It was found that RRIO op amps are useful for low-side current sensing. Difference amplifiers, however, can be used for high-side current sensing in the presence of high common-mode voltages at the expense of drawing current from the system bus supply. They can also be used for low-side sensing and negate the effects of parasitic resistances to ground. Instrumentation amplifiers, like op amps, are limited to common-mode voltages within their power supplies and are generally used for low-side sensing.
Finally, CSMs employ unique input stage topologies that allow for a wide range of common-mode voltages while placing less of a load on the system than a DA. Most CSMs can be used for either high or low-side sensing. Table 1 provides general solution recommendations based on the device’s supply voltage (Vdevice) with respect to the system’s bus voltage, or Vcm, and the magnitude of the load current.
In the next article, we will examine the accuracy of a solution. Specifications such as common-mode rejection ratio (CMRR), power supply rejection ratio (PSRR), and initial input offset voltage (Vos) will be introduced.
For answers to current sensing applications questions please visit TI’s Precision Amplifiers forum in the E2E community.
About the Authors
Peter Semig is an Applications Engineer in the Precision Linear group at TI where he supports difference amplifiers, instrumentation amplifiers, and current shunt monitors. Peter received his BSEE and MSEE from Michigan State University, East Lansing, Michigan. If you have questions about this article, contact Peter at firstname.lastname@example.org.
Collin Wells is an applications engineer in the Precision Linear group at TI where he supports industrial products and applications. Collin received his BSEE from the University of Texas, Dallas.