# Neophyte Question: measuring open circuit voltages

Discussion in 'Electronic Basics' started by [email protected], Dec 15, 2005.

1. ### Guest

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
0. How can I make this happen?

2. ### The PhantomGuest

Your NI board input circuitry has a bias current, probably in the nanoamp
region, which is charging up the cable. You could try injecting a current from
a stable DC voltage of opposite polarity through a *very* high value resistor,
adjusting the voltage and resistor until the voltage on the cable is nearly
constant. Then flex the cable and take your measurements quick like a bunny.

Or, get one of these:
http://www.national.com/ds.cgi/LM/LMC6001.pdf

and use it as a buffer to get the input current down to the femtoamp range. You
then momentarily short the leads to reset the voltage to zero, and the bias
current of the LMC6001 will charge the cable so slowly that you should be able
to get your measurements.