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

Mike Degerstrom ([email protected])
Fri, 10 Apr 1998 11:39:33 -0500


I've had a similar problem when trying to measure IC interconnect
test structures with an HP4285A. I believe that the HP4285A and
HP4275A operate on the same principles.

My problem was that it was very difficult to measure small values
of capacitance with the presence of DC leakage. This caused
my capacitance values as reported by the HP4285A to be too
high as the meter (apparently) cannot effectively discern
between capacitance and DC leakage.

In general, digital IC inputs have diodes and/or resistors shunted
to the power or ground paths for ESD, N/C logic state control,
clamping over/undershoots, and so on. So perhaps the
HP4285A cannot resolve the desired AC parameters for modeling
an input pin.

You may help your diagnosis of the problem by trying to measure
the resistance of the structure instead of the capacitance. In
general, I think that (1/jwC) must be much less than the leakage
or bias resistances to be able to measure the capacitance of a
general structure. Most meters also have some sort of bias
capabilities and it could prove useful to try measurement over
different bias levels.

I don't think that my solution of the problem was very practical. I
took HP8510C VNA s-parameter measurements of my structures
then converted to these to fit either pi or tee networks, whichever
made most sense for the structure. Then by taking 20*log(magnitude)
and phase of impedances that comprise pi or tee networks and
plotting vs. log(freq) , I could easily verify that I was measuring
either R,L, or C. For a R/L/C should have slopes of 0/+6db per
octave/-6db per octave and phases of 0/+90/-90 degrees, respectively.

So in general, I think the LCR meters work fine as long only
one parameter dominates but it seems that these meters cannot
measure general impedances. I have had luck with impedance
meters that provide me with magitude and phase. However, you
then have to compute an equivalent circuit model from your
measurements as I had described above.

On Apr 10, 10:38am, Greg Edlund wrote:
> 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?

Mike Degerstrom                       Email:    [email protected]	
Mayo Clinic - Gugg. Bldg. RM 1011-B    Phone:    (507) 284-3292
Rochester, MN 55905                    FAX:    (507) 284-9171