[SI-LIST] : measuring CMOS Cin with HP4275A

Greg Edlund ([email protected])
Fri, 10 Apr 1998 10:38:53 -0400

I have a board in the lab with a test structure for measuring the
capacitance of a CMOS input on a powered-up component. I'm using an
HP4275A, which is an LCR meter that utilizes an auto-balancing bridge
circuit. I can make the measurement with the board isolated from any
power source, but as soon as I even connect the board to a powered-off
supply, the meter reads overflow. It sounds like I have some kind of a
ground loop problem. Am I trying to do something that this meter was
not designed to do?

If you could lend your experience, it would be much appreciated. It's
for a good cause, too. When we finish this board, we'll be posting the
Gerbers and schematics to the IBIS web site. The purpose of the board
is to demonstrate how to make measurements that verify an IBIS
datasheet. (It's also useful for verifying SPICE models). Thanks in

Description of my test set-up:

There are three SMA connectors on the board: one for open cal, one for
short cal, and one that goes to the DUT. They each have an identical
piece of etch connected to the SMA center pin.

The probes are home-built from two dual-conductor, shielded cables. On
the instrument side, one of the cables is connected to Hp and Hc through
a two isolated pieces of copper foil mounted on perf board. The other
cable is connect to Lp and Lc in the same manner. The shields are
connected to chassis ground.

On the DUT side, the Hp and Hc conductors of one cable are connected to
the SMA center pin. The Lp and Lc conductors of the other cable are
connected to the SMA ground. The shields are connected to each other
but floating.

Greg Edlund, Principal Engineer
Server Product Development
Digital Equipment Corp.
129 Parker St. PKO3-1/20C
Maynard, MA 01754
(978) 493-4157 voice
(978) 493-0941 FAX
[email protected]