ZHCSMK4A september 2022 – july 2023 ADS131B26-Q1
PRODUCTION DATA
In a typical BMS application, the current through the shunt resistor must be measured in both directions for charging and discharging the battery pack. In an overcurrent or short-circuit condition, the current can be as high as IBAT_MAX = ±3 kA in this example application. Therefore, the maximum voltage drop across the shunt is up to VSHUNT = RSHUNT × IBAT_MAX = 50 μΩ × ±3 kA = ±150 mV.
To measure this shunt voltage, ADC1A is configured for gain = 8, which allows differential voltage measurements of VIN1A = VCPA – VCNA = ±VREFA / 8 = ±1.25 V / 8 = ±156 mV. The integrated charge pump in the device allows voltage measurements 312.5 mV below AGNDA while using a unipolar analog power supply. This bipolar voltage measurement capability is important because one side of the shunt is connected to the same GND potential as the AGNDA pin of the ADS131B26-Q1, which means that the absolute voltage that the device must measure is up to 150 mV below AGNDA.
To enable fast overcurrent detection within 1 ms while providing high accuracy and resolution, the ADS131B26-Q1 is configured to operate at 4 kSPS (OSR = 1024)) using global-chop mode. Global-chop mode enables measurements with minimal offset error over temperature and time. The conversion time using these settings is 0.75 ms according to Equation 21. The input-referred noise is approximately 1.28 μVRMS / √2 = 0.91 μVRMS following the explanations in the Global-Chop Mode section. Thus, currents as small as 0.91 μVRMS / 50 μΩ = 18 mA can be resolved. The resolution can be further improved by averaging the conversion results over a longer period of time in the microcontroller that interfaces with the ADS131B26-Q1.
The –3-dB corner frequency of the differential antialiasing filter on the analog inputs (R13, R14, and C6) is set to 1 / (2 × ᴨ × 2 × 100 Ω × 47 nF) = 16.9 kHz to provide more then 40-dB attenuation at the ADC1A modulator frequency. Keep the series resistor values (R13 and R14) small to avoid additional offset errors created by the voltage drop across the resistors because of the ADC1A input currents.
ADC1B is configured identical to ADC1A to allow for simultaneous sampling of the shunt voltage with the same digital filter response.