[sdiy] mosfet/cmos opamp

Ian Fritz ijfritz at comcast.net
Thu Oct 28 23:17:56 CEST 2010

At 01:14 PM 10/28/2010, David G. Dixon wrote:
>Yes, but the charging time also depends on the on-resistance of the switch.
>I'm using DG409, with a typical on-resistance of 40 ohms.  Assuming a
>charging time of 2.2RC, then in my case it's about 2.2*40*100/1,000,000,000
>= 8.8us -- call it 10us.  Even at the stated maximum of on-resistance of 100
>ohms, it's still less than 25us.  (Is this correct?)

I look at it differently.  I think of a current source driving the cap, so 
the time is ~CV/I.  In my case I'm driving at a bit under 1 mA (driven from 
an OTA used as a current switch) and the numbers look about right.  So I 
would look at what the current limit of your source is.  This may be where 
my thinking is wrong; you may have a lot more drive current than what I'm 
thinking of.


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