Putting it all together
As mentioned earlier, each of the error terms can be combined to determine a system’s overall accuracy. Simply adding each term yields the worst-case error while combining them in a RSS fashion yields a more probable scenario.
For the Vos, CMRR, and PSRR examples given in this article the worst-case accuracy would be 67.4% as shown in Equation 7.
Combining the errors in a RSS fashion yields a more probable accuracy of 78.03% as shown in Equation 8.
This article defined the concept of current sensing accuracy as it relates to specifications such as Vos, CMRR, and PSRR. Examples were given on how to calculate errors caused by these specifications in a particular application. Other errors such as gain error, shunt tolerance, and Vos shift and drift were also briefly discussed. The final article in this series will discuss current sensing PCB layout and troubleshooting guidelines.
1. Karki, “Understanding Operational Amplifier Specifications,” White Paper: SLOA011, Texas Instruments, 1998.
2. Tobey, Graeme, Huelsman, “Operational Amplifiers: Design and Applications,” McGraw-Hill, 1971.
3. Franco, “Design with Operational Amplifiers and Analog Integrated Circuits,” 3rd Edition, McGraw-Hill, 2001.
4. Frederiksen, “Intuitive Operational Amplifiers: From Basics to Useful Applications,” Revised Edition, McGraw-Hill, 1988.
For answers to current sensing applications questions, visit TI’s Precision Amplifiers forum in the E2E community: www.ti.com/e2e-ca.
The complete series
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 email@example.com.
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, Texas.