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3.用导联电缆屏蔽层反馈电路消除屏蔽层对输入阻抗的短路作用.前面已经提到,为了减少50赫电场所致的干扰,可考虑用带屏蔽层的导联线.但屏蔽导联线存在着分布电容,一条3米长的屏蔽线大约存在300PF左右分布电容.这相当于在放大器输入端并联上这样数值的电容,造成实际阻抗的降低,以致影响放大器对电位分压器效应的抗御能力.特别是当输入电缆线进一步加长时这种影响会更大一些.导联电缆屏蔽层反馈电路正是为解决这一矛盾而提出的.图七是将共模信号反馈到导联线屏蔽层的简化电路.
3. Use the lead cable shield feedback circuit to eliminate the short-circuit effect of the shield on the input impedance.As mentioned earlier, in order to reduce interference caused by the 50 Hz electric field, consider the use of shielded lead wire. However, the shield lead There is distributed capacitance in the line, there is about 300PF around a 3m long shielded cable distributed capacitance, which is equivalent to the capacitor input in parallel with such a value of the capacitor, causing the actual impedance decreases, so as to affect the amplifier on the potential divider effect Especially when the input cable is further lengthened when the impact will be greater.It is the lead cable shielding layer feedback circuit is proposed to solve this contradiction.Figure VII is the common mode signal is fed back to the lead wire Shield simplified circuit.