使用电压放大器或锁相放大器时，通常需要选择输入信号的耦合方式，这里介绍一些直流耦合和交流耦合的区别。
按照官方手册里的说明：
The COUPLING pushbutton selects the method of connecting the A and B inputs to the amplifier. The inputs can be AC (0.03 Hz – 3 dB) or DC-coupled, or the inputs to the amplifier can be internally grounded with the A and B input BNC’s left floating. This feature makes for simple offset nulling, particularly useful when operating the amplifier DC-coupled at high gains. Please refer to CALIBRATION AND REPAIR — OFFSET ADJUSTMENT for information on the offset nulling procedure.
NOTE: When the coupling is set to AC, a 0.03 Hz cutoff high-pass filter is always engaged. All high-pass filter modes can still be selected while AC-coupled, but the 0.03 Hz filter will always be in, even if the filters are set to DC. Because one of the two filter sections is always used as a high pass when AC coupling is selected, low-pass filters are only available with a 6 dB / octave rolloff.
This key selects the input coupling. The signal input can be either AC or DC coupled. The current input is coupled after the current to voltage conversion. The current input itself is always DC coupled (1 kΩ to virtual ground).
The AC coupling high pass filter passes signals above 160 mHz and attenuates signals at lower frequencies. AC coupling should be used at frequencies above 160 mHz whenever possible. At lower frequencies, DC coupling is required. AC coupling results in gain and phase errors at low frequencies.
Remember, the Reference Input is AC coupled when a sine reference is used. This also results in phase errors at low frequencies.
直流耦合就是直接的导线连接，包括通过像电阻之类的线性元件的连接。
它适用于对包括直流分量的信号的放大电路中。在直流耦合电路中，各级电路的静态工作点是互相影响的。
一级的工作点改变了相邻的二级也会受到影响。因此不能单独地调整工作点电流和电压。
而在交流耦合直流不耦合的电路中各级电路是用电容或者是电感隔离开的。
因此静态工作点是独立的，调整静态工作点比较容易。直流耦合中因为各级的输入和输出阻抗是一定的，不好作阻抗变换，直接耦合时高效率匹配就很难做到。
而在交流耦合电路中用线间变压器就很好地进行阻抗变换实现高效率的匹配。特别是选频放大电路中普遍采用的LC谐振电路更是极大地提高了电路的效率。
参考：
入門篇-耦合Coupling AC/DC/GND差別在哪
直流耦合-百度百科
