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From: Joe G (Home) on 14 Oct 2006 10:34
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?


Regards
Joe



From: John Popelish on 14 Oct 2006 11:28
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.
From: Jim Thompson on 14 Oct 2006 11:24
On Sat, 14 Oct 2006 11:28:57 -0400, John Popelish <jpopelish(a)rica.net>
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
--
| 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 |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
From: mkaras on 14 Oct 2006 11:42

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.
>
>

Capacitance measurements may also be influenced by the series ON
resistance of the switch. The Ron of a switch can vary depending upon
the voltage level of signal so you may also need to take this factor
into account.

- mkaras

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

From: Joe G (Home) on 14 Oct 2006 12:24
<snip>>
> 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.

Yep, I like it! Like a Weatstone Bridge arrangement.....


In Telecom POTS.... to convert 2wire to 4 wire... a Weatstone bridge
arrngement is often used to cancel the local Tx audio from going in to the
Rx local.

http://en.wikipedia.org/wiki/Wheatstone_bridge

Regards
Joe


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