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HCPL-7800 isolation amplifier motor current sensing application

HCPL-7800 and MCP3301 DC motor isolated current sensing circuit

HCPL-7800 is an isolation amplifier by Broadcom inc or AVAGO Tech. AVAGO Tech use optical isolated sigma delta modulation to translate input signal. Using optical isolation there is no leakage current between GND1 and GND2 even at full bandwidth operation.

HCPL-7800 bandwidth is 100KHz which is almost double to capacitive barrier technology based isolation amplifiers.

HCPL-7800 need 5V supply for input circuit and output circuit.

In following HCPL-7800 example circuit we control a DC motor by MOSFET MCU20P10-TP switching by micro controller via 4N35 opto-coupler. We can control motor speed by applying square wave with different duty cycles to opto coupler.

However, to judge mechanical load on motor we need to measure motor current. R1 is shunt resistor in series of motor. Voltage across R1 are directly proportional to motor current. which are 10mV on 1A motor current. HCPL-7800 Vin+ and Vin- tied to GND1 pins are parallel to R1. Voltage across to R1 will appear on output side of HCPL-7800 Vout+ and Vout- pins. This output is connected to ADC MCP3301 differential analog input via two 2KΩ resistors. 2.5V Reference voltage are applied by MAX6125 3-pin voltage reference IC. Further more MCP3301 13bit ADC sample propagate serially to micro-controller with synchronize by clock input from micro-controller. micro-controller can start acquisition by CS pin and CLK pin of ADC. MCP3301 maximum sampling frequency is 100KS/s.

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