[SI-LIST] : update on measuring CMOS Cin with HP4275A

Greg Edlund ([email protected])
Tue, 21 Apr 1998 09:51:30 -0400

I want to thank all those who took the time to reply to my message a
week ago. There were some really intriguing suggestions. I especially
liked the TDR method since it eliminates one piece of test equipment; I
think I'll look into this further.

I've done some more lab work since I posted the first message. We
reworked our home-built probes to enhance their mechanical stability. I
am now able to measure capacitance with the board powered up, but I
don't believe the results because I can't calibrate the instrument. It
calibrates fine when the board is floating, but not when I connect it to
a power supply (even with the line switch off). The LCR meter starts
it's open cal procedure at 10 MHz, begins stepping down in frequency,
and gives me an "Err1" message at 2 MHz.

I went back and re-read my "Impedance Measurement Handbook" from HP (a
good piece). The auto-balancing bridge instruments have an internal
"virtual ground" at the Lp/Lc terminals, and I'm suspicious that this
ground is conflicting with the power distribution looking into the test
board, although I can't prove this. I'm going to pursue one more
suggestion for using the HP4275A in this application, but if it doesn't
work, I'll go after one of the others you all mentioned.

Thanks again,

From: Greg Edlund[SMTP:[email protected]]
Sent: Friday, April 10, 1998 8:38 AM
To: '[email protected]'
Subject: [SI-LIST] : measuring CMOS Cin with HP4275A

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]