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Since the high voltage battery can reach up to 150V, we must either select op-amps with a very high common voltage input, which is very difficult to find, or generate a reference voltage 3.3V or 5.0V below the HV battery voltage. This can be either accomplished by biasing a zener diode or by using an isolated DC-DC power supply with a floating ground below the HV battery voltage. Otherwise, the circuit is a fairly standard differential amplifier feeding into a buffer amplifier to increase it's output resistance, and this analog signal is sampled by a 24-bit sigma-delta ADC for maximum precision. The ADC should output the sampled value over SPI, and this signal will be translated into isoSPI using a LTC6820 IC, where it is sent to the BMS carrier board through a transformer for isolation. On the BMS Carrier board, there is another transformer and LTC6820, where the isoSPI signal is translated back to normal 4-wire SPI and read by the controller board. This is summarized in the diagram below, which omits any filtering that has to be designed on the input side.
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Part Selection
Current Sense Amplifier
With a 0.1mOhm shunt resistor and currents between -200A to 200A, the output sense voltage will be between -20mV and +20mV. As most amplifiers are rated for +3.3V or +5V operation, 5V will be used as it will allow the largest voltage range. Using a 2.5V reference, we are able to sense bidirectionally, as currents between -200A and 0A will be between 0V and 2.5V, and current between 0A and 200A will be represented with voltages between 2.5V and 5V. This means that 5V/40mV = 125V/V is the maximum gain we can have from our current sense amplfier. With bidirectional current sensors, typically only gains of 50, 100, and 200V/V are avaliable, so we will use the 100V/V gain variant. The other requirements for the current sense amplifier is a low offset voltage, high CMRR, and low drifts across a wide temperature range. The TI INA240A3 fits this description and will be used on this board.
Power Consumption
| Component | Power Supply | Voltage (V) | Current (mA) |
|---|---|---|---|
| INA240 Current Sense Amplifier | LDO | 5 | 2.6 |
| LM3120 4.096V Reference | LDO | 5 | 0.275 |
| TLV316-Q1 GP RR Op-Amp | LDO | 5 | 0.575 |
| LTC2484 ADC | LDO | 5 | 0.25 |
| LM3120 4.096V Reference | LDO | 5 | 0.275 |
| LTC6820 isoSPI Interface | LDO | 5 | 5.8 |
| LED | LDO | 5 | 1.5 |
| Total | 11.275 |
Filtering
To be tested.