Re: CMOS analogues switch capacitance

On Sun, 15 Oct 2006 04:09:17 GMT, Robert Baer
<robertbaer@xxxxxxxxxxxxx> wrote:

Jim Thompson wrote:

On Sat, 14 Oct 2006 11:28:57 -0400, John Popelish <jpopelish@xxxxxxxx>
wrote:


Joe G (Home) wrote:

Hi All,

I am measuring capacitance in the pF range and I need to switch monitoring
inputs.

We can calibrate out any constant capacitance.. However

The 64,000 dollar question is would you expect the input capacitances of the
CMOS switch to be constant over tempreature?

The devices I have looked at are the better versions of the CMOS 4053 etc
... a few from NXP (formerly Philips)
http://www.standardics.nxp.com/products/switches/


While there are input capacitance specifications..... there is no spec on
the relationship between input capacitance vs Temp.


Would you expect input capacitance to be fairly constant over the operating
temp range?

The short answer is that, unless someone has taken great
pains to suppress it, everything changes with temperature.

I would use an extra section of switch to measure the
capacitance of a switch, and subtract the changes of that
that from all the other measurements.


John, _Very_good_suggestion_!

The only relatively constant term would be gate capacitance, ESD and
MOS body diode capacitances will be all over the place with
temperature.

...Jim Thompson
DEfine "gate capacitance"; i think it is subject to changes over
temperature.

Duh! The gate oxide is GLASS... so its changes are quite small.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
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