Neophyte Question: measuring open circuit voltages

Hi,

I'm trying to measure the voltage induced by a peizoelectric cable (applying pressure to cable creates voltage/current on cable). But when I measure the voltage across the cable I get a "background" signal that looks like an exponentially decaying voltage, like a large capicitance discharging. I can measure the real voltage across the cable, it appears on top of the decaying voltage, but soon the voltage reaches what I assume is the max low value for my board (National Instruments, BNC 2120, floating point measurement), -10 volts. I can get the voltage to go back to zero by touching the leads but this just sets the process in motion again. This decaying voltage characteristic is also present when nothing is connected to the leads, which makes sense, hence my question about measuring open circuits. Needless to say I would like the voltage of an open circuit, or accross my cable to be

1. How can I make this happen?

Thanks in Advance.