Re: [sdiy] Re: Daffy CMOS ASCIImatic

[jhaible at debitel.net]

>This is good info.  And it brings me to a question.  Considering
>the 4016 analog switch, the spec claims that a signal of 5v p-p
>is usable with Vdd-Vss of 10v, for 0.4% "distortion".

I think the distortion figure will only make sense if a load resistance
(after the switch) and/or a source resistance (in series with the switch)
is specified. The THD just comes from the R variation due to the
voltage variation across the chip. Simplifying a little, one could
say there would be almost zero THD if your load resistance is
high enough. But if you increase the load resistance, you will get
more bleedthru from the capacitance across th eswitch when it's
*open*, so you have to find a compromise that fits your application.
There are better switches, too (see Maxim fo rinstance), but for
many applications the 4016 or 4066 will be fine. I may be wrong,
but I don't believe the 0.4% THD are still valid if you switch 
a 220k resistor to an opamp's virtual GND summing node.

>They seem to omit a noise spec altogether for the 4016. 

I meant the noise of an amplifier, built from a 4007 inverter with
feedback resistor. I don't expect noise to be an issue for a
4016 switching 1Vp signals. 
BUT - if you choose a load resistance of 10M to keep THD
low, you might get a noise problem from this *resistor* when
the switch is turned *off*. Unless a second switch connects
another low impedance source (or simply GND) to the load
resistor (1p2t configuration).


>Why do they recommend using a signal as large as half the rail 
>to rail span?  
>Is this because in the 4016 it is expected that the pass through 
>transistors are either full on or full off?

Yes. No linear operation here.

>So it appears that the _4016_ (not my morphodite 4007 thing) can
>work with a fairly large input signal...  This spec is why I 
>thought this might work.  But I really don't know what happens
>distortion/noise wise when the control signal is continuous and 
>not 2 state digital.

I don't know either. Well, a 4016 will not work. But whether a transistor
from a 4007 in linear VC-resistor mode is more noisy than a BF245
I don't know. (Even though this is what I expect.) What I know is that
two transistors from a 4007 in linear *amplifier* mode are more noisy
than a TL071 opamp. But as I said, this is just from undocumented
experience, and part of it may be because you can run a 071 with
10k feedback resistors, while a 4007 amplifier normally needs higher
values (= more noise).

JH.