Signal processing device, liquid crystal display and test system of liquid crystal display

A signal processing device, a liquid crystal display device, and a test system of the liquid crystal display device are provided to protect the LCD(Liquid Crystal Display) device from an excessive current by disconnecting an external voltage source from the LCD device, when the LCD device is defective. A signal processing device(10) includes an external signal supply unit(100), a memory(240), and a test unit(230). The external signal supply unit supplies an external clock signal and an external driving voltage and outputs a test signal. The memory stores reference data corresponding to the external clock signal. The test unit compares the external clock signal with reference data by using a checksum of the external clock signal and feeds back the test signal which represents whether the external clock signal is received without errors to the external signal supply unit.