Mixed Mode (Digital / Analog) trouble

[Sparky44]

Greetings all,

Well, I am sure it's probably me, but am stumped on what I need to do to fix my problem.  I have a circuit which uses a monostable multivibrator which is (i believe) considered an analog component, as well as then Digital components (binary up/down counter, Hex Inverter, AND,...etc). 

The issue I am having is trying to utilize the pulses generated by the multivibrator with the digital components.  Looking at the output of the multivibrator when digital simulation settings are Ideal (fast), I dont see an anomaly with the multivibrator output.  However, the logic analyzer does not capture any state transitions for that output.  When looking at the same point with the O-Scope I see the pulsed waveform I expect. 

To troubleshoot this I changed the Digital Simulation settings to Real then looked at the waveform with the O-Scope.  I found that the output pulses were to the correct peak amplitude but were riding on 2.5 VDC, a floating ground issue.  I cannot find a way to resolve this.  I've added and tied together analog and digital grounds (multisim dosent like that), with no effect.

The circuit is attached

HELP....

[Sparky44]

Well....I found a work around to this issue by using a DAC.  Once processed through that device, the expected output results.

[Sparky44]

Currently I am ROFL at myself.  Well....my last comment was a bit premature....was looking at the wrong node on the logic analyzer.  I did use a TTL compatible optocoupler for the ADC which seems to have quashed the floating DC level, but for reasons unknown the resultant output from the AND to the logic analyzer and thus the counter still does not display or react as expected.  Using the O-Scope I can clearly see the output of the AND trigger correctly and produce an output strobe at the appropriate time.

So....still am absolutely confused....time for a break....

[lacy]

I have taken a look at this and I believe I have found at least a solution for 2 of the problems you are seeing.

1)  The reason I believe the Logic Analyzer is not picking up on the last two pulses is that the clock frequency is exacly timed with the transistion. I ran the Analyzer with the internal clock and increased the frequency to 10Khz. This allowed me to see all the pulses.

2) When measuring with the O-scope you have a signal riding on 2.5V DC. I did see this.The problem is you are referencing to the analog ground. When measureing Digital component outputs (like the AND Gate), your refernce has to be a DGND or the results will be goofy.

I believe the above to be what is happening, but there could be other explainations for what is occurring. These are only my observations and I hope it helps.

[Sparky44]

Lacy,

You are absolutely right.  I took a long break yesterday before working it again and low and behold I found a few things that I have done wrong.  For one, being so intent on trying to figure out why I could not see the pulse, I failed to notice that I had connected the AND to the wrong input on the counter (CET instead of Load).  Furthermore, I had neglected to wire the parallel inputs to ground to facilitate a reset of the counter anyway.  Because of this I was holding the count for the short duration of the pulse from the AND which of course is why I was not seeing the correct circuit response (reset) and ultimately got me running down ghosts.

Thanks for your help!

[lacy]

O.K. I thought there was a problem with that, but I wasn't sure so I didin't say anything. In the original schematic it appears to me that the AND is actually tied to the D input of the 74191 and the load input is Tied back to VCC. Anyway, I think between us we have made it work and that is all that matters.

Sometimes I have problems getting the connections to where I need them. I think it is failure to pay attention to what I am doing and then when the circuit doesn't work I get really ticked off and want to throw the computer out the window until I realize that it was me that caused the problem to begin with. Things like this happen.Thankfully due to the simulator, no components were actually harmed in the making of my circuit.

[Sparky44]

Lacy,

Indeed, no damage to electronic components, less of course the computer keyboard and a few bruises on  the forehead. 

Attached is the completed circuit.  The objective was to take a serial data input which contains muxed DC representations (by pulse width) timed via a strobe.  I built this to facilitate decoding an MSI Seven Band Graphic Equalizer IC (MSIGEQ7) which in turn will drive a bar graph display.  Attached is the Datasheet for the device along with this completed circuit.  

[kkwan]

Hello all,

Greetings!

Now, I am encountering a similar problem that discussed in the below email threads in a few months back.  When the Digital Output in attached circuit (sorry, MultiSim 9, I will have MultiSim 10 soon) was measured in Logic Analyzer or the Agilent O-Scope, I didn't see any output displayed neither.  Please advise if any thing implemented wrong with this setup. 

Thank you so much for the help in advance.

K. Kwan