设计一个MCS-51单片机控制的程控增益放大器的接口电路。输入信号小于10mv,要求当输入信号小于1mv时，增益为1000，而输入信号每增加1mv时，其增益自动减少一倍，直到100mv为止。(Design a MCS - 51 single chip microcomputer control the interface circuit of programmable gain amplifier.Input signal is less than 10 mv, asked when the input signal is less than 1 mv, gain of 1000, every increase 1 mv and the input signal, the gain automatically to reduce doubled, until 100 mv.)