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This paper presents a principle of analog to digital conversion (ADC) based on a current mode circuit without DAC. For example, in this circuit the input value can be converted to a 4 bit output at each moment, and multiple output bit numbers by serial connection. In this current mode, the active current mirror and current comparators control the reference current by adjusting the W/L ratio. Its feasibility agrees with simulation results by the PSPICE program. The circuit design used a CMOS 0.5 μm process which is capable of converting 4 bits in 50 ns, with a power consumption of 0.127 mW, input current of 0-100 μA and single 3 V supply. From simulation testing, the conversion rate is faster than other methods using the same parameters.