# Reducing cable capacitance

1. Aug 14, 2009

### enamul101b

I am trying to build a Schering bridge to measure capacitance change for my research. I need to reduce the cable (line) capacitance to a minimum. I am using an ac 4V (rms), 250 KHz supply.

I tried to use two seperate coaxial cables to connect the capacitor plates. The signal goes through the inner cable and the outer wrap wire is grounded in both cables (this should work as Faraday cage). But unfortunately I see the line capacitance increases for that.

Please suggest me what should I do to minimize the line capacitance.

Thanks in advance for helping me out.

Kabir

2. Aug 14, 2009

### Bob S

Use the highest impedance and the highest signal velocity coaxial cable you can get. This based on the following two equations for coax cables:

Cable impedance is Z = sqrt(L/C)
signal velocity = βc = 1/sqrt(LC)

where L = inductance per meter, and C = capacitance per meter.

3. Aug 14, 2009

### Staff: Mentor

To minimize the capacitance, you can "bootstrap" the shields. Drive each side's shield with a buffered version of the test signal. This basically eliminates the capacitive current to charge up the wire with respect to the shield in each coax.