本帖最后由 airwill 于 2018-2-27 10:52 编辑
Signal Chain Basics #93: How to Maximize Low-Side Sensing Performance
What is low-side current sensing?
The most common method of measuring current is low-side sensing. A sense element or shunt resistor is placed in series with the load between the load and ground (Figure 1).
Figure 1
Simplified circuit diagram configured for low-side current-sensing.
The drawbacks to low-side sensing are disturbances to the system load's ground potential and the inability to detect load shorts to ground. Low-side sensing is desirable because the common-mode voltage is near ground. With common-mode range at or near zero, low-side sensing can be implemented using traditional operational amplifiers (op amps), instrumentation amplifiers (INAs), difference amplifiers, or current-sense amplifiers (or current-shunt monitors).
Simple operational amplifier
Using an op amp for current sensing is limited by the input common-mode voltage which is limited by the supply voltage. The large open-loop gain requires feedback, which limits its use to single-ended input signals. Parasitic resistance between the shunt resistor and ground trace adds to the shunt resistor value (Figure 2). The voltage developed by the parasitic resistance will "pedestal" the shunt voltage, introducing error. This parasitic resistance may vary greatly in production. For greater accuracy and consistency, a differential measurement across the shunt resistor is required. While the op amp solution most likely is the lowest-cost option, it offers the lowest accuracy -- unless significant money is spent on the external components, offsetting the device cost savings.
Figure 2
Example circuit for using a simple op amp for low-side current sensing, highlighting the error introduced by parasitic resistance.
Difference amplifier
A traditional difference amplifier (DA) is simply an op amp with a precision-trimmed resistor network (Figure 3). DAs typically are designed to drive large signals with unity gain. This can limit their usefulness with small signals typically seen across the shunt resistor, unless additional gain stages are added, resulting in additional cost.
A DA is seen as 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. To reduce 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 0 V, this minimizes the effect of the common-mode input impedance, but the differential-mode input impedance is still a factor. Using a DA for low-side measurements negates the issue caused by parasitic resistance-to-ground in series with the shunt resistance as discussed by the op amp.
Figure 3
Example circuit for using a difference amp for low-side current sensing, eliminating the error introduced by parasitic resistance.
Instrumentation amplifier
Instrumentation amplifiers (INAs) typically are composed of a DA output stage with buffered inputs (Figure 4). The first advantage over a DA is that the inputs are connected to the non-inverting inputs of a buffer amplifier, which are high-impedance. This translates to almost no load on the system voltage, enabling the sensing of small currents. Also, INAs offer the ability to easily change the device's differential gain using an external resistor (RG). However, to maximize accuracy, a precision resistor is needed, which is costly.
Additionally, for low-side sensing applications, the three-stage INA has an internal node on the output stage that typically mandates dual-supply implementations, potentially driving up system cost. With three stages and integrated precision resistors, INAs tend to be large devices that cost more.
Figure 4
Example circuit for using an INA for low-side current sensing, eliminating the error introduced by parasitic resistance.
Current-sense amplifier/current-shunt monitor
A current-sense amplifier (CSA) is a device that places little load on a system and allows for sensing current under high common-mode voltage conditions through the design of unique input stages. Current-sense amps are small, low cost, and simple to use. They include all the gain setting resistors and are specified for maximum gain error -- eliminating many external accuracy factors. Due to its differential input structure and zero-drift architecture, a current-shunt monitor eliminates wiring and PCB layout parasitic resistance. This improves performance and reduces errors across the full temperature range as the parasitic layout and wiring resistances are copper and have a high-temp coefficient.
A current-sense monitor has very high input impedance and a very high common-mode rejection ratio (CMRR), which is critical to maximize accuracy. Due to the accuracy enabled by the CSA's architecture, smaller-value shunt resistors can be used to achieve the same results as the above alternatives. Using a smaller shunt minimizes load loss, as well as heat in the system.
Early current-shunt monitors were aimed only at high-side applications and did not include 0 V in their common-mode range. Most new devices include ground in their common-mode range, such as the INA199 family, which has a common-mode range from –0.3 V to +26 V, enabling its use on the low side. It has a typical offset voltage of ±5 µV with a temperature drift of only 0.1 µV/°C. Combine this with a typical gain error of ±0.03%, again with near zero temperature drift of 3 ppm/°C.
Summary
While there are many alternatives for low-side current measurements, current-shunt monitors offer significant accuracy advantages, lower power consumption, and smaller footprints. They enable the use of smaller shunt resistors, in many cases at equal or lower cost.
Table 1
Various low-side sensing alternatives.
Download the INA199 datasheet.
Join us next time, when we discuss key considerations in selecting an anti-aliasing filter for SAR ADC applications.
Dan Harmon is Sensing Business Development Manager for TI's sensing group. In his 27+ year career at TI, he has supported a wide variety of technologies and products, including interface products, imaging analog front-ends (AFEs), and charge-coupled device (CCD) sensors. He also has served as TI's USB-IF Representative and TI's USB 3.0 Promoter's Group Chair. He earned a BSEE from the University of Dayton and an MSEE from the University of Texas in Arlington. You can reach him at ti_danharmon@list.ti.com.
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