[sdiy] 4051 spikes

[Richie Burnett]

It's probably just down to stray capacitance in the CMOS switch.  CMOS 
switches are internally made out of MOSFETs and they have stray capacitance 
between their terminals.  It looks like you are just looking at the output 
with a scope probe (possibly x10) which will have a very high impedance 
(with no additional load?), so it will tend to respond to any edges that AC 
couple through the switch's stray capacitances.  Does the behaviour improve 
if you put a 10k load resistance at the output across the scope probe?

This "glitch energy" behaviour is common in analogue multiplexer circuits, 
but the effect is often swamped by the relatively large size of the Sample 
and Hold capacitor.

It could be due to stray capacitance in the breadboard, but I doubt it.  Try 
removing the 4051 and see if you can still see the spikes!?

-Richie,

-----Original Message----- 
From: florian anwander
Sent: Saturday, April 25, 2015 10:38 PM
To: SDIY List
Subject: [sdiy] 4051 spikes

Hello,

I have setup today a simple test circuit for a batch of 4051
(HEF14051BCP to be exact), which I received for free. While testing I
found that all 4051 I own (not only the ones from the received batch)
show some short feedthrough of the common signal at outputs 3 and 5 at
not addressed times.
A picture of the scope image can be found here (signal from output 3)
http://fa.utfs.org/diy/CD4051_1.jpg
The test setup can be seen here:
http://fa.utfs.org/diy/CD4051_2.jpg
It is simply a 4069 as oscillator, a 4520 as address counter and the 4051.
One has to turn down the clock rate into the audiorange, to make this
vanish.

Is this normal? Is it caused by the breadboarding setup?

Thanks for any insight.

Florian
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