Amplification Gain

The gain of the amplifier influences different parameters and thus the system performance. The previous BMS current sensing system uses 100 V/V. Here, I will examine other Gains to see if we can improve the system by changing the amplification gain. I will compare between 100 V/V, 50 V/V and 200 V/V. All the calculations are for the INA240 current sensing amplifier and Vout of 4.5V.

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Note: ENBW is obtained from TIDA-03050 for ADS1259 for worst-case scenario.

By using 200 V/V instead of 100 V/V, we can reduce the power dissipation in the system by 50% (higher efficiency) and reduce the self-heating in the shunt. Reducing the resistance value also reduces the cost and size of the resistor and the board as a whole. Moreover, resistance affects accuracy through temperature as the temperature drift changes the nominal resistance of the shunt.

As increasing the gain reduce the sensed voltage drop across the resistance, INA240 enables current sensing with maximum drops across the shunt as low as 10-mV full-scale, which is the voltage drop for 200 V/V.

However, by reducing the resistance values and the thus the losses in the system, we encounter accuracy issues. Amplifier input offset voltage is the main source of error in low current levels. Therefore, minimizing this error is critical in the current-sensing design. As the amplification gain decreases, the offset error decreases. By using 50 V/V instead of 100 V/V, we reduce the offset error by 12.5%. This is a worst-case scenario with the lowest possible current (1 A).

We also need to consider noise measurement in low current levels where the voltage drop is very low. It becomes difficult to differentiate the signal and the noise as they are both in uV. The input-referred noise of an amplifier is the major contributor to noise in the system. Amplification noise is performed with respect to gain because the noise is multiplied by the gain. As shown, using 50 V/V instead of 100 V/V improves the noise accuracy by 40%.

Therefore, if we are considering replacing 100 V/V, we need to either maximize efficiency (200 V/V) or accuracy (50 V/V). We cannot enhance the system accuracy through the resistor’s tolerance and temperature co-efficient as they are almost the same for this range of resistance (As far as I concern). However, we can improve the system noise by reducing the sampling rate. Moreover, we can compensate for the temperature error of the resistor by using an LMT01 temperature sensor to detect the temperature of the shunt.