This article will investigate some standards that affect the accuracy of solutions. These specifications include input offset voltage (Vos), common mode rejection ratio (CMRR), and power supply rejection ratio (PSRR). The system accuracy of current shunt measurement is affected by too many error sources, including the error sources shown in Table 1. The worst-case system accuracy is defined as Equation 1. Among them is the error contribution rate (%) of each error source. However, a more likely way to calculate system accuracy is to combine uncorrelated errors in the form of square roots (RSS), as shown in Equation 2. Input offset voltage is usually the biggest factor affecting the accuracy of the solution. It is defined as "the DC voltage that must be applied between the input terminals to force the static DC output voltage to zero or other levels, if specified." [1] The ideal Vos of an amplifier is 0V. However, process changes and equipment design limitations can result in non-zero VO values. Calculate all input reference errors based on the ideal parallel voltage. The ideal parallel voltage is the product of the load current and the ideal parallel resistance value. Let's calculate the error contribution of a device's Vos specification in a system with a rated load current of 5A and an ideal parallel resistance value of 1m?, As shown in equation 3. Assuming we decide to use INA170 with a maximum Vos specification of 1mV.

To reduce this error, we have two options: increase the Rshunt resistor or decrease Vos (max). Due to cost, circuit board space, or power consumption, increasing Rshunt resistance may or may not be feasible. On the other hand, we can try to find a low VO alternative device. Finally, pay attention to the inverse relationship between load current and error. In our example, we calculated that 20% of the rated load current is 5A. If the system load current decreases, the contribution error caused by the Vos specification increases. Therefore, designers should calculate the worst-case error under the minimum load current.

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