Meaning
Electronic circuit technique amplifies the difference between two input voltages while rejecting common mode noise to ensure the integrity of sensor data in high EMI environments. This differential signal conditioning is essential for measuring the small voltage drops across current shunts and individual battery cells. It governs the accuracy of the analog front end in the battery management system.
The technique applies to the entire signal path from the sensor leads to the input of the analog to digital converter.
Noise Rejection
Common mode signals that appear equally on both input wires are cancelled out by the differential amplifier. This differential signal conditioning protects the measurement from the electromagnetic interference generated by the motor controller and the power inverter. The effectiveness of the rejection is measured by the common mode rejection ratio of the circuit.
High rejection is necessary to maintain signal integrity in the electrically noisy environment of an electric vehicle. Twisted pair wiring is often used to ensure that noise is coupled equally to both signal lines. Proper shielding of the cables further reduces the pickup of external interference.
Signal Amplification
Low level signals from thermocouples or current shunts must be increased to match the input range of the converter. This differential signal conditioning uses instrumentation amplifiers to provide high input impedance and precise gain. The high impedance prevents the loading of the sensor and ensures an accurate reading.
Gain errors and offset voltages must be minimized to maintain the calibration of the system. Temperature stable resistors are used in the feedback loop to reduce the drift over time. The circuit must also provide filtering to remove high frequency noise before the amplification stage.
Measurement Reliability
Integrity of the data is the primary concern for the safety functions of the battery management system. This differential signal conditioning allows for the reliable detection of overvoltage and undervoltage conditions. Faulty readings can lead to premature disconnection of the battery or the failure to detect a dangerous state.
The circuit design must include protection against transient voltage spikes and reverse polarity. Isolation barriers are sometimes required to protect the low voltage electronics from the high voltage battery bus. Consistency in the signal processing ensures that the balancing algorithms operate correctly.
The final processed signal provides a true representation of the electrochemical state of the cell.
