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cd4051 bug?

What is the load impedance? You can get capacitive feed-thru with high load impedance.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
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Reply to
Jim Thompson
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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?

Reply to
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?

Reply to
whoandcar

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?

Reply to
Robert Baer

YES: if one over-drives an input, it can latch - and if overdriven too much, then the input can act as if it is always Hi even if grounded.

Reply to
Robert Baer

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

Reply to
Roger Lascelles

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?

Reply to
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.

Reply to
whoandcar

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