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Keywords: current sense error, current sense accuracy, Isense error, Isense accuracy, CSA error, CSA accuracy, error
budget calculator, total error, system error budget, resistor tolerance, CMRR, input offset voltage, gain error, PSRR
APPLICATION NOTE 5095
DC Error Budget Calculator Simplifies Selection of
Optimal Current-Sense Amplifiers
By: Akshay Bhat, Senior Strategic Applications Engineer
Apr 09, 2013
Abstract:
This article
describes a systematic approach for calculating the total DC error budget of a current-sense
amplifier (CSA). The individual sources of error are discussed and a method for estimating the total error budget is
presented. Finally, a step-by-step process explains how to use calculator software, developed to help quickly compute
the total error of the selected CSA.
A similar version of this article appeared in German in the February 6, 2013 issue of Elektronikpraxis magazine.
Introduction
Integrated current-sense amplifiers (CSAs) are frequently used to measure the current flow in electronic circuits. They
amplify the small voltage drop across a sense resistor inserted in the current path to perform critical system-level
functions. Examples include overcurrent-protection and supervising devices, programmable current sources, linear and
switch-mode power supplies, battery chargers, and fuel gauges. While the required current-sense specifications and
implementation schemes are as diverse as these applications themselves, analysis of the CSA error budget is a
fundamental component of every design. A thorough understanding of CSA error specifications and their interactions
will prove immensely helpful when selecting the appropriate device for an application. Needless to say, that
understanding will also minimize last-minute design iterations.
This article discusses CSA error sources and outlines a methodology for estimating the total error budget. A step-by-
step process explains how to use calculator software developed by Maxim Integrated. With a simple web-based GUI,
this software determines the total DC error budget for any selected Maxim CSA. An application example will familiarize
the reader with the basic operation of the calculator. Design tips and message flags will alert you to any circuit
conditions that are inconsistent with the CSA's specifications.
Error Sources in a Current-Sense Amplifier
There are several sources of DC error prevalent in a CSA. Each is reviewed briefly.
Input Offset Voltage
Similar to an op amp, the input offset voltage (V
OS
) of a CSA is defined as the voltage that must be applied across the
CSA's inputs to drive the output voltage to zero. Offset errors are usually not measured directly, because in single-
supply situations the output of the CSA cannot go below the output voltage low (V
OL
) limit. Hence, the input V
OS
is
more accurately inferred as the intersection between the linear regression of the observed V
OUT
and the V
SENSE
transfer curve on the V
SENSE
axis as shown in Figure 1.
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