Here is an update.

After talking to people in class as well as the professor, this is what i need to do as of now.
1. I need to find a way to build a comparator by using transistors ?
2. If i am going to need a clock signal in which i am then i must build an circuit to do so.
3. I can use a d/a chip since the goal of the project is A/D, but i am having a bit of trouble finding a chip for this application. I was thinking of the DAC5571 but its package is too small, maybe the 558 ?
4. Also the Successive approx Register i was going to use is now obsolete (74LS503), any suggestion on substitutions ? 74C905N ?

Remember the Project constrains above, basically all discrete parts (ie resistors, transistors, and 74 series ICs).


I would like to order some stuff as soon as i can, so that i may get started any suggestions or help would be appreciated.

pete

 

A DAC is easy, try looking up "R2R ladder"

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Ok i got info regarding the R2R ladder and i guess that should work well . Any ideas schematics on building a comparator for this application ?

pete

 

I am not a analog person but I found this lie detector circuit that has a simple transistor comparator here. I do not know if it is sensitive enough or exactly what transistors to use. I am sure that one of the analog types will be able to suggest part numbers.


Comparators are often based on op amps, if you could use one of them it would be easy to do a first rate comparator with just a few parts.

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Well i got all the parts and i am starting to assemble the project on my breadboard.
Thanks 3vo for the comparator circuit. I am going to try to get it to work with the lm311 comparator chip then i will try to swap it out with an alternative circuit to get rid of the comparator IC.
I am trying to get it all connected but i cant seem to get the LM311 to work properly, i believe because it spec sheet refers to vee, which should be a negative voltage. I dont think that it can be just grounded, if not how do i create a negative voltage without another power supply ?

Also i should add that the 8 bit successive approximation register that was mentioned earlier is discontinued and i am now trying to use a 74c905 which is similar but a 12 bit version. Within its spec sheet, it has a crude version of the analog to digital project circuit...take a look...

thanks
pete

 

I tried using the LM311 in one of my circuits and although supposedly a simple device in theory, as a newbie to electronics I had some problems getting it to work properly. Here are some pointers.. .The LM311 will work with a single +5V supply ( Pin 8 ) and 0V ( Pins 1 and 4 ). Pins 5 and 6 should be shorted together. The inputs ( Pins 2 and 3 ) can range from 0v-5V ( i.e. rail-to-rail ) without adversely affecting the 311, BUT you must ensure that at least one of the inputs is at, or below 3.5V or switching won't occur! ( another head scratcher, 'til I found out about common mode voltages! ). Adding a bit of hysteresis wouldn't go amiss either and will help eliminate output oscillations around the switching threshold, but again watch that the varying switching threshold due to the added hysteresis doesn't wander above that 3.5V upper limit..!

 

Ooooh! If I were the student, I might get creative with using 4XXXB series CMOS in linear mode. Of course, I'd have to be allowed to use resistors.

Bob

 

Hey Bob, thanks for the suggestion but i am only allowed to use discrete parts such as resistors, diodes, caps and all 74 series family ICs. So i need to eventually make a comparator using those. For now untill i get it working i will use the comparator chip.

Thanks for the suggestions saiello i believe i have the lm311 working, it took a few shots to understand it but its ok now.

Here is an overall update with a few questions. Like i said before i am using the 74c905 12 bit succ approx register. I have it all connected as it suggests in the typical application parts of its spec sheet (i have a cropped link below). There are a few exceptions to the way i have it connected but i think that should not matter much..For one i am using the LM311 instead of the suggested MM54C909 and i am using a R=10k instead of 50k for the DAC ladder.

Using these components the A/D will not work unless i disconnect the OR gate (7432) and just connect the /S to /CC directly. If i do that by inputting a variable analog voltage i get various digital values outputting. Any ideas on why thats happening ? I blieve that OR should be there bc the fact i am using this chip in 8 bit mode not 12b ??

Anyway look at the spec sheet link and let me know guys...thanks again..
PEte

 

Why do you think pins 5 and 6 should be shorted together? This lowers the gain of the comparator by a factor of about 5, although it may speed it up a little. Otherwise, you have done a good job of deciphering the datasheet.
I'm assuming you used a pullup resistor from the output (pin 7) to +5V.

Amdkickass, have you used a pullup? It won't work without one.
Also - for dynamic input signals, you are going to need a sample-and-hold.
If you really get stuck on discrete circuit design (e.g., the comparator), I can help. For the comparator, it would be really helpful if you could use transistor arrays (e.g. CA3046). Can you, or would that be considered an IC?
I also think you will need positive and negative power supplies.

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Well Ron i really appreciate your response. The way i look at it, is i would prefer to get the project working pretty well with the lm311 chip and then eventually replace it with discrete components to satisfy the project design specifications. As far as transistors are concerned i can use all BJTs and MOs transistors. By doing it this way i can kinda get an idea of where i should be with values and what not.

On the output of the comparator , i have resistor connected to +5 ( thats the pull up ?). The main two problems i am having is that connecting the OR gate (as stated in the spec sheet) causes the circuit to not function, like i said before above. I really dont understand why that not working. The other problem i am having is that as i turn the pot to mimic various analog input voltages (10k pot) i get a serial digital stream (serial out of Succ approx Reg) viewable on the scope until i get to 3.3volts input approximately then all of a sudden the digital out goes all High. I am not sure but this may be associated to what you mentioned above dynamic analog input , but i think the output shouldn't go high until i approach 5 volts (the reference voltage).

THanks
pete

 

As Saiello pointed out, the common mode range of the LM311 only goes up to about 3.3V with a 5V supply, and down to about 0.5V above the negative supply (GND in your case). You will probably have to get a CMOS comparator to get rail-to-rail input range, and then I'm not sure if you can get one that's fast enough, although it might be for proof-of-concept. I think I would bite the bullet and go with multiple supplies. Actually, considering that you are having to use discrete-component circuits (comparator, sample-and-hold, etc.), I would probably use around +/-9V to +/-12V for my supplies, with a 5V regulator (do you have to use a zener for this?) running off the positive supply for the logic and the reference.

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Ron (aka Rube)

 

Well currently i am using a +12 power supply and everything in my circuit is running off a 5 volt regulator (i believe its a 7805).
I am sorry i am not sure if i understood you 100%. Are you saying i need to supply the lm311 with a negative voltage or rather replace it with another unit ? Because if supplying it with a negative voltage wouldn't be a problem if that would take care of that issue.

pete

 

On the LM311, connect pin 8 to +12V, pin 4 to a negative voltage (any voltage between -1V and -15V), pin 1 to GND, and your output pullup resistor to +5V (as you already have it connected). Leave pins 5 and 6 floating. Your input will then work over the full range of 0 to 5V.
You should decouple the power supplies on pins 4 and 8 with 100nF caps to GND, as close to the pins as possible, and with leads as short as possible (surface mount caps are best, but not required). In fact, you should decouple the power on each IC this way, and also the discrete circuits, when you get them built.

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Ron (aka Rube)

 

After connecting the LM311 like you said plus messing with the D/A Ladder i got it to work across the full scale.
The next problem is its inaccuracy ? What i mean by inaccuracy is the fact that the guess voltage thats being spit out of the D/A is about.3V off the actual input voltage. I believe this is happening because the the fact we are only using 8 of the 12 bits in the conversion process. I think the spec sheets takes care of that by using the OR gate (see attatched link above), but like i said connecting it causes the circuit to simply not work at all any ideas ?

Thanks
pete

 

What is the part number of your OR gate? Is it a 74C32? What D/A are you using?

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