cd4051 bug?

[whoandcar]

I made a project using a 4051 to select one of 5 inputs. I take the output
fro pin 3.
I used addresses 1, 2, 3, 4, and 5. Some times I have signal out when
there is no signal at input 4 but there is some on input 3. It is kind of
a crosstalk. But it only happens between inputs 3 and 4 and never in
others. I have made about 1 hundred devices and had problems say in 15-20
of them. Is that a bug of the 4051? Some one heard of something similar?

 

[whoandcar]

Load impedance = 10 K R to ground - Frequency = fix @ 5 KHz - Definitely
not capacitive coupling. All 5 channels are equal,the 5 signals are the
same - only jumps from input #3 to #4 - strange, isn't it?

 

[Roger Lascelles]

"whoandcar" <rey@lanet.com.ar> wrote in message
news:cd88d6be32a57c63576c6147dc230ee4@localhost.talkaboutelectronicequipment
..com...

Load impedance = 10 K R to ground - Frequency = fix @ 5 KHz - Definitely
not capacitive coupling. All 5 channels are equal,the 5 signals are the
same - only jumps from input #3 to #4 - strange, isn't it?

Millions of 4051 chips have worked as expected, so I doubt you have
discovered a design bug.

Suggestions :

A batch of weird chips ?

You are pushing logic or analog pins too high or low and getting
interconnection via substrate current flow ? For example, driving from
opamps with more swing available than the 4051 power rails.

The VDD, VSS, VEE voltages are not what you think they are ?

Roger

 

[Robert Baer]

whoandcar wrote:

I made a project using a 4051 to select one of 5 inputs. I take the output
fro pin 3.
I used addresses 1, 2, 3, 4, and 5. Some times I have signal out when
there is no signal at input 4 but there is some on input 3. It is kind of
a crosstalk. But it only happens between inputs 3 and 4 and never in
others. I have made about 1 hundred devices and had problems say in 15-20
of them. Is that a bug of the 4051? Some one heard of something similar?

Aer all inputs terminated with the appropiate HI or LOW levels and not
open wen you move the signal from one input to the other?

 

[whoandcar]

I know that if you drive the input past 0.6 Volts over Vcc or minus 0.6 V
under Vee it will latch or crosstalk. That is not the case. Vcc is +5.0
coming from a 78L05, digital lines come from a PIC 16F84A at +5.0 Volts
also.
On analog lines I tried everiything form biasing them at 0 Volts to
coupling by capacity to let if self-bias. No results.
Forgot I mention that all 5 signals are identical?

 

[whoandcar]

Resolved. Tanks to you all, friends.
As always the problem was out of the circuit and is clear now. I used a
set of pre-amps to amplify the signal of photo-diodes. On some sets, the
boys (installers) used to run the wires of the input to one pre very near
the output of another pre. Simply basic cross-talk. And always happened on
channel four... because of the topography.