- #1
edmondng
- 159
- 0
Hi,
I'm trying to measure some low capacitance using parallel plates i built. We're probably looking into the pF, say maybe 20pF (with the pcb board etc). The change in capacitance that i am really interested is very small. For the moment, i have use water as my dielectric. The capacitance that i am measuring is measured in terms of voltage by using a control capacitor, then goes through my 16bit A/D. The differential voltage from 0ml to 250ml is approx 20mV. This is a really small value.
One thing i did notice is that there seems to be some sort of fringing effect field surrounding the plates. Put you hand within 1 feet of the plates and the voltage swings away. Stay 3 feet away and its more stable. The stability increases with distance, so the nearer you are the more the voltage swing. I take it because the capacitance and e-field is being disturbed, small capacitance change in the pF results in my voltage change.
I was thinking of using some kind of guard material to isolate my test area. What sort of material should i use? Plastic doesn't seem to have any effect. Also since i am looking at small values, is it possible to see a bigger change or amplify my signal? Its 16bit A/D, amplifying the voltage would not help. What i need is a bigger change like from 0ml to 250ml with maybe 1V change rather than 20mV. Any help, suggestion, books to look at would be appreciated.
Thanks
I'm trying to measure some low capacitance using parallel plates i built. We're probably looking into the pF, say maybe 20pF (with the pcb board etc). The change in capacitance that i am really interested is very small. For the moment, i have use water as my dielectric. The capacitance that i am measuring is measured in terms of voltage by using a control capacitor, then goes through my 16bit A/D. The differential voltage from 0ml to 250ml is approx 20mV. This is a really small value.
One thing i did notice is that there seems to be some sort of fringing effect field surrounding the plates. Put you hand within 1 feet of the plates and the voltage swings away. Stay 3 feet away and its more stable. The stability increases with distance, so the nearer you are the more the voltage swing. I take it because the capacitance and e-field is being disturbed, small capacitance change in the pF results in my voltage change.
I was thinking of using some kind of guard material to isolate my test area. What sort of material should i use? Plastic doesn't seem to have any effect. Also since i am looking at small values, is it possible to see a bigger change or amplify my signal? Its 16bit A/D, amplifying the voltage would not help. What i need is a bigger change like from 0ml to 250ml with maybe 1V change rather than 20mV. Any help, suggestion, books to look at would be appreciated.
Thanks