Title: [Entry] Discrete logic preamp
Post by: haley0918 on October 20, 2011, 05:50:36 am
Post by: haley0918 on October 20, 2011, 05:50:36 am
Here’s my first entry for the 7400 competition. This is a discrete logic preamp using SN74LS86AN from Texas Instruments. I was focusing on the misuse of 7400 logic chips of this competition and thought of using TTL to do analog signals.
It happens that I have a SN74LS86AN XOR chip on my table and I began the testing by hooking up a pot to one of the inputs while tying the other high. Through the scope, I find that the XOR gate does show analog property. However, the analog property is only limited to a region where the input ranges from about 0.8v to 1.2v with the output swings from about 1v to 4v. Since this chip had been sitting on my desk for quite some time, I thought that it might have had electrostatic damage. So I bought another same chip for test and to my amaze, the new chip also shows the same property. Below is a footage showing my test of both old (left) and new (right) chips. Apologize for the blurriness as the video was captured using my phone.
http://http://www.youtube.com/watch?v=Ek82heftfmc
I went on to order a few other 7400 TTL (not CMOS) logic chips with different logic gates and none of them demonstrate the analog property. Perhaps the manufacturers had optimized the chips to only switch the output to either high or low only when reaching the thresholds. I also tested another 74F86 and the analog property was observed.
Based on the result and observation above, I came up with this discrete logic preamp. Below are the schematic and board design for the preamp. All four XOR gates in the chip were brought out so totaling four preamps in a board. Each of the inputs was tied to a trimmer for offsetting the input voltage to about 0.95v and about 2.5v at the output. Another important thing to note is that the circuit has to be exactly in the arrangement shown to reproduce the result. The inputs and trimmers have to be on pins 2, 5, 9 and 12 of the chip. The other input of each gate also has to be tied high.
Here's the link to video showing the final board
http://http://www.youtube.com/watch?v=I_8rtkv5PRY
It happens that I have a SN74LS86AN XOR chip on my table and I began the testing by hooking up a pot to one of the inputs while tying the other high. Through the scope, I find that the XOR gate does show analog property. However, the analog property is only limited to a region where the input ranges from about 0.8v to 1.2v with the output swings from about 1v to 4v. Since this chip had been sitting on my desk for quite some time, I thought that it might have had electrostatic damage. So I bought another same chip for test and to my amaze, the new chip also shows the same property. Below is a footage showing my test of both old (left) and new (right) chips. Apologize for the blurriness as the video was captured using my phone.
http://http://www.youtube.com/watch?v=Ek82heftfmc
I went on to order a few other 7400 TTL (not CMOS) logic chips with different logic gates and none of them demonstrate the analog property. Perhaps the manufacturers had optimized the chips to only switch the output to either high or low only when reaching the thresholds. I also tested another 74F86 and the analog property was observed.
Based on the result and observation above, I came up with this discrete logic preamp. Below are the schematic and board design for the preamp. All four XOR gates in the chip were brought out so totaling four preamps in a board. Each of the inputs was tied to a trimmer for offsetting the input voltage to about 0.95v and about 2.5v at the output. Another important thing to note is that the circuit has to be exactly in the arrangement shown to reproduce the result. The inputs and trimmers have to be on pins 2, 5, 9 and 12 of the chip. The other input of each gate also has to be tied high.
Here's the link to video showing the final board
http://http://www.youtube.com/watch?v=I_8rtkv5PRY