Re: CMOS analogues switch capacitance

mkaras 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.




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


Not a significant factor; what is worse in using a FET as a switch for commutating an input to a mesuring system, is the fact that the applied gate voltage will show up (attenuated by capacitive division) in the source and drain.
.

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