[sdiy] 4051 spikes

[Richie Burnett]

...could also possibly be something caused by ground-bounce.  I don't see 
your scope-probe grounding crocodile clips clipped onto the ground rail of 
your breadboard :-p

The digital waveforms have quite a lot of tilt on them too, which suggests 
either bad decoupling or ground bounce.

I don't know the time-base setting for these scope captures but you said the 
switching frequency was above audio range.  When dealing with fast-edged 
digital signals above audio frequencies, you should have supply decoupling 
capacitors across the supply rails close to the logic ICs, and connect your 
scope probe grounds to the circuit ground via the shortest possible route. 
These two tips help to make sure that you see a genuine representation on 
the scope of what is actually going on in your circuit.

(It's maybe also worth checking the compensation of your scope probes too, 
while you're at it, if they have one of those little HF adjustment screws 
somewhere?)

Hope this helps,

-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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