In a message dated 97-11-07 02:09:37 EST, you write:

<< My problem is that these resistors are not very common and
 rather expensive (I need 2 because each unit will have
 both a positive and a negative high voltage). >>

Ruben,
I assume you are going to pot this circuit as surface leakage will be a very
large parasitic when you are using 500-1000M resistors. In either case, you
could cut your cost in half by using a single resistor (2x the resistance)
between the plus and minus voltage. I am interested to know how you make
out...good-luck.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel: 716-328-5850
Fax: 716-328-1144
http://www.frontiernet.net/~rmi/
***** Designing Electronics for Research and Industry *****