0
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

I was trying to recreate the following ADC (using comparators or NAND/NOR logic elements) circuit to control a 74HC4514 with a 0-5V voltage, but for some reason it doesn't behave as it should.

http://zpostbox.ru/simple_adc.html

Looks like the outputs of the ADC aren't either ON or OFF, but transition between those states and messing up the stages on the 74HC4514. I tried to do the ADC circuit with a 74HCT00, 74HCT02 and 74HCT04, but all with the same result. When using Op-amps as comparators, all outputs are either ON or OFF.

There must be something I'm missing in this simple circuit, any ideas?

\$\endgroup\$
  • \$\begingroup\$ Is this for a steam project? \$\endgroup\$ – Voltage Spike Apr 5 '16 at 4:51
  • 1
    \$\begingroup\$ Please provide a schematic, use the schematic editor... \$\endgroup\$ – Voltage Spike Apr 5 '16 at 4:52
  • \$\begingroup\$ It's just for a small personal project. I'm using the same exact schematic as the link is referring to. I don't know what else to draw that can give some added value. \$\endgroup\$ – Richard W Apr 6 '16 at 15:37
1
\$\begingroup\$

You cannot make an ADC with logic gates. Your approach to use op-amps was the correct one. If it didn't work surely there must be some mistake there. Try to test your comparators one by one, to see that they work OK, each one for its intended range.

You could even start with a '1 bit converter' where a comparator divides the analog input range to > Vcc/2 and <=Vcc/2. Then move on to the 2 bit ADC.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ I tried that and all comparators are working perfectly on their own and thus also the 1 bit converter. When I extend it to a 2 bit I get the same problem as before with all bits. \$\endgroup\$ – Richard W Apr 6 '16 at 20:43
  • \$\begingroup\$ It's also not really clear to me how he circuit behaves. The top comparator OA1 outputs high when the input voltage at the + input is >2.5V, but before that, I see no reason for the other comparators to go high. And with a high output of OA1 and thus and input voltage of 2.5V, the node between R1 and R4 should also at least be +2.5V triggering a high output of OA2 and keeping it high fom there on until the overal input voltage is <2.5V, right? Should I implement pull-up/down resistors and if so, where and what should be the recommended values? \$\endgroup\$ – Richard W Apr 6 '16 at 21:01
0
\$\begingroup\$

I think you attempted to make a comparator circuit, the image below shows the differences. What you have essentially puts the opamps in positive feedback and will cause them to swing to one rail or the other. You might be able to accomplish what you intended with comparator circuits, if you make a comparator, you can have no feedback from either terminal to the input. If you intended to do something else, like attenuate the output, then make sure the op amp is in negative feedback.

schematic

simulate this circuit – Schematic created using CircuitLab

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.