Spehro Pefhany wrote:

> http://www.fairchildsemi.com/ds/MM/MM74HC4053.pdf
>
> +/- 7.5 seems to be abs. max.
>
> The on resistance is highest well inside the Vcc- Vee voltage range, and
> is lowest at the rails, right where you'd use it. Typical is more like
> 20-30 ohms, you can probably scale that safely by the typ:max ratio.
> See the curve on page 7.

I was looking at the TI version, this Fairchild part is considerably
better, and in fact meets the specs for what I need.  A couple of these
does the trick.  Thanks.

> I'll leave those calculations to you, as I don't know your
> application.

The output drives a PIN diode RF switching circuit that uses the 20 ma
to get the ON resistance of the PIN diodes down to a fraction of an ohm.

Driver -> RF Choke -> PIN Diode -> Coax -> PIN Diode -> RF Choke -> GND

The two diodes drop about 1.2V total, so 3.8 volts/20ma = ~200 ohms
series resistance needed.  This would be split between the ON resistance
of the output FET and a series resistor.  If the ON resistance of the
FET is only 30 ohms, then this is only 15% of the total, and part
variances would be limited to affecting the current by only this amount.
If the ON resistance were 160 ohms (TI part worst case), then part
variances would affect the current dramatically.

The -5V reverse biases these same PIN diodes, as well as forward biases
a different set of PIN diodes acting as shunts across the coax to
provide even further isolation.

-Johnathan

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body