RE: [SI-LIST] : measuring CMOS Cin with HP4275A

Shenoy, Jay ([email protected])
Fri, 10 Apr 1998 14:46:03 -0700

Hi Greg,

The measurements I set up with an HP4275A were about 4 years ago,
and there is a crucial
principle of operation "schematic" that I can recall only with partial
certainity. In that context,
(& with the disclaimer above of less-than-perfect recollection of
details of operation), it seems to
me that you might want to excahnge the Hp/Hc pair with the Lp/Lc pair,
i.e. connect the
Hp/Hc to ground and Lp/Lc pair to the SMA center pin.

When you are conducting measurements on isolated DUT's , it is
possible to gloss over
the difference between the Hp/Hc and Lp/Lc pairs, which explains the
success of the first of
your measurements. However, (if I remember correctly) the Hp has a AC
voltage source and
there is an AC ammeter through the Lp/Lc pair, so putting a big
capacitor on Hp/Hc (e.g.
you connecting your power supply) just means that the AC voltage source
has to feed that
capacitor too,& if you don't see the AC voltage source dropping
precipitously in magnitude on the front
panel (somewhere on the right side) , the measurement per-se has not
been affected. However,
if you put the same capacitor (as you seem to have) on Lp, you've just
provided an alternate
current path for DUT current, which was supposed to flow through the AC
ammeter for correct

The only point where my hypothesis does not match up with what you
say, is that it predicts
reading an _underflow_ and a lot of fluctuation on the reading. But it
is worth the try, since the
suggestion I have (exchange the Hp/Hc with Lp/Lc) is very simple...

Jay Shenoy

> -----Original Message-----
> From: Greg Edlund [SMTP:[email protected]]
> Sent: Friday, April 10, 1998 7:39 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
> advance.
> 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]